Help Center/ Object Storage Service/ User Guide/ Data Management/ Using Mirroring-Based Back to Source to Retrieve Data

Updated on 2026-08-21 GMT+08:00

Using Mirroring-Based Back to Source to Retrieve Data

Usually, if the data requested by a client using GET is not found in OBS, a 404 error will be returned. To avoid this error, OBS provides back to source that pulls the requested data from the source site (origin server) if it is not found in OBS.

Process of Back to Source by Mirroring

If the data you requested is not found in a bucket that has a back-to-source by mirroring rule configured and the request matches the rule, OBS will pull the data from its origin server and return it to you. This process does not interrupt services. Therefore, you can use this function to seamlessly migrate data from the origin server to OBS without being sensed by users, at low costs. #EN-US_TOPIC_0000001858185970/en-us_topic_0000001874795321_en-us_topic_0129288983_fig8517161311401 shows the process of back to source by mirroring.

Figure 1 Process of back to source by mirroring
Click to enlarge

During the back to source by mirroring process, OBS functions as a proxy. However, if the files on the origin server change after they are stored in OBS, OBS will not automatically update the files.

Back-to-Source Rules

Item	Description
Conditions for triggering an origin pull rule	OBS pulls data from the origin server only when a GetObject request results in a 404 response.
Rules for naming the retrieved files	If OBS retrieves a file from the origin server via http(s)://MirrorURL/ObjectName, the file is stored in OBS under the same name, ObjectName. For example, if the source address set for a bucket is https://yun.com and the requested file example.jpg is not found in the bucket, OBS will retrieve the file via https://yun.com/example.jpg and file example.jpg is then pushed to OBS.
Status code return rules in the case of retrieval failures	If OBS fails to retrieve a file from the source, it returns 404 to the client by default.
Rules for updating the retrieved files	If a file has been pushed by the origin server to OBS through mirroring, OBS does not update the file even if the source file changes.
HTTP request rules	By default, headers and query strings included in requests to OBS are not transferred to the origin server. However, you can configure the back to source settings to specify whether they should be passed along.

Constraints

**Table 1** Back to source constraints
Category	Description
Bucket versions	Only buckets of version 3.0 or later support back to source by mirroring.
Time	It takes about five minutes to apply any changes to a back-to-source by mirroring rule.
Number of rules	A maximum of 10 back-to-source by mirroring rules can be configured for a bucket.
Functions	Anonymous users are not allowed to configure back-to-source by mirroring rules. Federated users cannot use back to source by mirroring. For more information about federated users, see What Are the Relationships Between a Huawei Cloud Account, HUAWEI ID, IAM User, and Federated User? and Introduction to Identity Providers. Parallel file systems do not support back-to-source by mirroring rules. Static website hosting does not support back to source by mirroring. Specifically, if 404 is returned when you use a static website domain name to download an object, back to source by mirroring will not be triggered. A bucket cannot mirror itself. Currently, back to source by mirroring from private buckets is only supported for some cloud vendors. In back-to-source setups, two buckets cannot act as each other's origin. For example, if bucket A is the origin for bucket B, bucket B cannot be the origin for bucket A. Transfer-Encoding: chunked cannot be used for the origin server to transmit data, or the origin pull will fail. The response to the request for downloading an object from the origin server must contain the Content-Length header to specify the size of the source object. To achieve this, in the Create/Edit Back-to-Source Rule window, choose Pass all parameters for HTTP Header Pass Rule, enable Do not pass specified parameters, and add Accept-Encoding. If both a CDN acceleration domain name and a back-to-source by mirroring rule are configured and range requests are enabled on CDN, the origin server must return at least one of the Etag and Last-Modified headers. Otherwise, the request may be interrupted. If NGINX was deployed as a reverse proxy for your origin server, turn off chunked_transfer_encoding of the NGINX. location / { chunked_transfer_encoding off; }
Permissions	To configure, obtain, or delete back-to-source by mirroring rules, you must have the Tenant Administrator permission assigned through role/policy-based authorization of IAM. You must create a cloud service agency using role/policy-based authorization of IAM to delegate OBS to pull data from the origin server. The agency must include the obs:object:PutObject, obs:object:GetObject, obs:bucket:ListBucket, and obs:object:AbortMultipartUpload permissions. If SSE-KMS is enabled for a bucket, the kms:cmk:get, kms:cmk:list, kms:cmk:create, kms:dek:create, kms:dek:encrypt, and kms:dek:decrypt permissions must be configured for the IAM agency for OBS using role/policy-based authorization.
Others	Back to source by mirroring is free now. An object cannot match two different back-to-source by mirroring rules.

Important Notes

If a file has been pushed by the origin server to OBS through mirroring, OBS does not simultaneously update the file even if the source file changes. You can use the following methods to indirectly synchronize files in OBS with those on the origin server:
- Manually deleting pulled files: Manually delete the files cached in the bucket using the console or APIs. This ensures that the system fetches the latest data from the origin server on the next pull.
- Configuring lifecycle rules to periodically delete pulled files: Configure lifecycle rules for pulled files so that they are automatically deleted after a specified period. This ensures that the system fetches the latest data from the origin server on the next pull.
- Naming origin server files by version: When origin server files are updated, use version names or timestamps (for example, testv1.txt or test20260617181156.txt) to distinguish them. This helps quickly identify updated files.
When using back to source mirroring for data migration, ensure that the origin server has sufficient bandwidth to handle back-to-source requests. You are advised to migrate data during off-peak hours and monitor the origin server load in real time.
During back-to-source, the system continuously fetches data from the origin server and stores it in the bucket, which will incur storage charges. You are advised to set an expenditure alert to keep storage costs under control.
If the source address uses the HTTPS protocol, ensure that the certificate configured for the origin server is trusted and valid.

Creating a Back-to-Source by Mirroring Rule

You can use OBS Console, APIs to create back-to-source by mirroring rules. You cannot use SDKs, obsutil, or OBS Browser+ to do so.

Using OBS Console

In the navigation pane of OBS Console, choose Buckets.
In the bucket list, click the desired bucket. The Objects page is displayed.
In the navigation pane, choose Data Management > Back to Source. The back-to-source rule list is displayed.
Click Create.

Configure Back-to-Source Condition.

**Table 2** Back-to-source condition parameters
Parameter	Description
HTTP Status Code	If the retrieval from the source fails, OBS returns a 404 error to the client.
File Name Prefix	Enter a prefix to specify which files will trigger the back-to-source rule when they are requested but missing from the bucket. The file name prefix: Cannot exceed 1,023 characters. Cannot contain or overlap with any other file name prefix specified for an existing rule. Can be left blank, which means that the rule applies to all files that do not meet the conditions of other back-to-source rules configured for the bucket. A bucket can have only one prefix-free back-to-source rule. Example: If you set the file name prefix to file/test, the back-to-source rule will be triggered whenever a requested file matching file/test is not found in the bucket.

Configure Source URL.

Click to enlarge

**Table 3** Parameters under Source URL
Parameter	Description
Prefix and Suffix	Whether OBS adds a prefix or suffix to the name of the object requested by the client when OBS requests data from the origin server. Skip: OBS requests data from the origin server using the object name requested by the client. Add prefix or suffix: OBS adds the specified prefix or suffix to the name of the object requested by the client. However, the object returned to OBS and the client retains its original name, without the added prefix or suffix. If you select this option, you also need to set a prefix, suffix, or both. The prefix and suffix: Cannot contain special characters like \:?"<>\| Cannot have two consecutive slashes (//). Example: The prefix is set to 123. A request for abc.txt* triggers OBS to pull 123abc.txt from the origin server. However, the object is saved in OBS and returned to the client under its original name abc.txt. Replace prefix: OBS uses the characters specified here to replace the file name prefix defined in the back-to-source condition. However, the object returned to the client keeps its original name and does not include the replacement prefix. Example: The file name prefix is set to 123 as the back-to-source condition and abc is used to replace the prefix. In this case, a request for 123456.txt triggers OBS to fetch abc456.txt from the origin server. However, the object is saved in OBS and returned to the client under its original name 123456.txt.
Source Site Type	The type of the origin server Public: The origin server data comes from public object storage and can be accessed through a public network domain name. Private: The origin server data comes from private object storage of certain cloud vendors.
Active Site	The active site of the origin URL. During the back-to-source process, OBS preferentially pulls data from the active site, or polls all active sites if multiple are available. If two or more active sites are configured, and if the first request to an active site fails while the retry conditions are met, OBS will attempt to pull data from another active site. If all attempts on the active sites fail, OBS will try the URLs of the standby sites. The constraints on configuring active sites are as follows: Configure at least one active site. Configure a maximum of five active sites at the same time. In back-to-source setups, two buckets cannot act as each other's origin. For example, if bucket A is the origin for bucket B, bucket B cannot be the origin for bucket A. Format: http://Source domain name/Static path or https://Source domain name/Static path Set the protocol to http or https. Set the source domain name to the domain name or IP address of the origin server. It can be an origin server IP address but must be a public IP address. If the origin server is a bucket accessible over HTTP, use the bucket's domain name as Source domain name. If the origin server is a private bucket hosted by another cloud vendor, use the regional domain name as Source domain name. At present, only private buckets of certain cloud vendors are supported. If the origin server is a public or private OBS bucket on Huawei Cloud, query the bucket and region domain names by referring to Accessing OBS. For details about how to query the public or private bucket domain names and region domain names on other cloud vendors, see the related documents of the cloud vendors. Set the static path to the directory where the target file is located. Example: If the static path is set to 123/, the target file is in the 123/ directory.
Standby Site	The standby site of the origin URL. During the back-to-source process, OBS preferentially pulls data from the active site, or polls all active sites if multiple are available. If two or more active sites are configured, and if the first request to an active site fails while the retry conditions are met, OBS will attempt to pull data from another active site. If all attempts on the active sites fail, OBS will try the URLs of the standby sites. The constraints on configuring standby sites are as follows: Configure a maximum of five standby sites at the same time. In back-to-source setups, two buckets cannot act as each other's origin. For example, if bucket A is the origin for bucket B, bucket B cannot be the origin for bucket A. Format: http://Source domain name/Static path or https://Source domain name/Static path Set the protocol to http or https. Set the source domain name to the domain name or IP address of the origin server. It can be an origin server IP address but must be a public IP address. If the origin server is a bucket accessible over HTTP, use the bucket's domain name as Source domain name. If the origin server is a private bucket hosted by another cloud vendor, use the regional domain name as Source domain name. At present, only private buckets of certain cloud vendors are supported. If the origin server is a public or private OBS bucket on Huawei Cloud, query the bucket and region domain names by referring to Accessing OBS. For details about how to query the public or private bucket domain names and region domain names on other cloud vendors, see the related documents of the cloud vendors. Set the static path to the directory where the target file is located. Example: If the static path is set to 123/, the target file is in the 123/ directory.
Source Bucket Vendor	Object storage service provider of the origin server address. If the origin server type is set to Private, you need to select the source bucket vendor for both the active and standby sites. Source Bucket Vendor: The provider of the source bucket. The currently available options are Alibaba, ByteDance, and Huawei Cloud. Source Bucket Name: The name of the private bucket that functions as the origin server. AK: The AK of the account that owns the bucket acting as the origin server. SK: The SK of the account that owns the bucket acting as the origin server. If the source is an OBS bucket on Huawei Cloud , obtain the AK/SK by referring to Obtaining Access Keys (AK/SK). For details about how to query the public or private bucket domain names and region domain names on other cloud vendors, see the related documents of the cloud vendors.

Configure Pull Settings.

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**Table 4** Parameters under Pull Settings
Parameter	Description
Retry Condition	Condition when a retry is triggered. If the origin server returns specified status codes, the request to the origin server is retried. Status codes starting with 4 or 5 are supported. A maximum of 20 status codes can be selected. 4XX and specific status codes starting with 4 cannot be selected at the same time. The same applies to 5XX and specific status codes starting with 5.
Include request strings	After this option is enabled, query parameters in the request URL will be passed to the origin server.
Redirect requests to the returned location	When this option is selected, if redirection is configured for the origin server, the resources will be requested from the location included in the 3xx response returned by the origin server and be saved to OBS. Each request supports up to 10 redirects.

Configure HTTP Header Pass Rule.

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**Table 5** Parameters under **HTTP Header Pass Rule**
Parameter	Description
HTTP Header Pass Rule	You can specify the HTTP header parameters that can be passed to the origin server when a request sent to OBS triggers the back-to-source by mirroring rule. Retrieving Data from the Origin Server and Passing Specified Client Parameters to the Origin Server provides a configuration example for your reference. Pass all parameters/Pass specified parameters: Set the HTTP header parameters that can be passed. Do not pass specified parameters: Set the HTTP header parameters that cannot be passed. If a client's request includes the specified headers, OBS will not pass them to the origin server. If a header is specified for both the pass and do-not-pass categories, it is deemed as a do-not-pass parameter. Configure custom parameters: You can set a custom value for a specified header. If a client request includes that header, OBS overrides it with the custom value before passing it to the origin server.

Notes for passing HTTP headers during back to source

The following HTTP headers can be passed from an origin server to a client: Content-Type, Content-Language, Content-Encoding, Content-Disposition, Cache-Control, and Expires

The following HTTP headers cannot be passed from a client to an origin server:

HTTP headers starting with the prefix x-obs-
All standard HTTP headers, for example, Content-Length, Authorization, Range, and Date

Others

**Table 6** Parameters under Others
Parameter	Description
IAM Agency	An IAM agency is required to delegate OBS to fetch data from the origin server and store the data in OBS. The agency must include the obs:object:PutObject, obs:object:GetObject, obs:bucket:ListBucket, and obs:object:AbortMultipartUpload permissions. If no suitable IAM agency is available, click View Agencies to create an agency on the IAM agency page. For details, see Creating an Agency for Back to Source.
Redirect requests	After a redirection address is configured, if OBS fails to pull the requested resource from the origin server, the origin server returns a 302 status code along with the redirection address to the client. The client then requests the resource from that address. Match: Forward match: Back to source by redirection will be performed if the returned status code is any of those you specified. Reverse match: Back to source by redirection will be performed if the returned status code is not among those you specified. Redirection URL: http://Source domain name/Static path or https://Source domain name/Static path A source domain name is the domain name of the origin server to which requests are redirected. Static path indicates the directory that stores the target file. For example, if the static path is 123/, the target file is in the 123/ directory.
Transparently pass error messages (when HTTP 4XX status codes returned)	If the origin server returns a 4XX status code, OBS will transparently pass response headers and error messages to the client.
Redirect requests without including Referer	If redirection has been configured for the origin server, enabling this option will remove the Referer header from requests during the redirection process.
Disable active health check	By default, active health checks are performed. If you do not need them, select this option. OBS periodically sends health check requests to the origin server to determine whether the service is running properly based on the responses.

Click OK. After the rule is created, it appears in the list on the back to source page.

Using APIs

Creating a Back-to-Source by Mirroring Rule

Replicating Back-to-Source by Mirroring Rules

You can OBS Console to replicate back-to-source by mirroring rules. You cannot use APIs, SDKs, OBS Browser+, or obsutil to do so.

In the navigation pane of OBS Console, choose Buckets.
In the bucket list, click the desired bucket. The Objects page is displayed.
In the navigation pane, choose Data Management > Back to Source. The back-to-source rule list is displayed.
Click Replicate.
Select a replication source, which is the bucket whose back-to-source rules you want to replicate.

Figure 2 Replicating back-to-source rules
- The back-to-source rules replicated from a source bucket do not overwrite existing rules in the destination bucket. Any rules that conflict with existing ones are not replicated.
- Both source and destination buckets must be of the 3.0 version. For details about how to query the bucket version, see Checking Your OBS Version (OBS 2.0 or OBS 3.0).
- Before replication, you can change the source URL. For details, see Table 3.
- You can remove the rules that you do not want to replicate.
- A bucket can have up to five back-to-source rules. If the number of rules you plan to replicate plus the number of existing rules in the destination bucket exceeds five, the replication will fail. Before replicating the rules, delete some if necessary.
Click OK to replicate the rules to the destination bucket.

Back to Source by Mirroring Examples

Obtaining Missing Files from a Specified Website

In this example, when a user accesses the test-file/test.txt file in the test-bucket bucket in the CN North-Beijing4 region, the file is automatically retrieved from the origin site https://testexample.com/.

Step 1 Configure a back-to-source rule for bucket test-bucket.

In the navigation pane, choose Data Management > Back to Source. The back-to-source rule list is displayed.
Click Create.

Configure the parameters as described in the following table. Keep the default settings for other parameters.

Click to enlarge

**Table 7** Parameters
Parameter	Value Example	Description
File Name Prefix	test-file/	Prefix of the file to be retrieved. In this example, set this parameter to test-file/, which indicates that the rule is triggered when the requested file is not found in the test-file folder in the bucket.
Active Site	Protocol: https Source domain name: testexample.com Static path: Leave it blank.	The active site of the origin URL. Format: http://Source domain name/Static path or https://Source domain name/Static path Set the protocol to http or https. Set the source domain name to the domain name of the origin server. Set the static path to the directory where the target file is located on the origin server. Example: If the static path is set to 123/, the target file is in the 123/ directory on the origin server.

Click OK.

Step 2 Verify the rule.

Access https://test-bucket.obs.cn-north-4.myhuaweicloud.com/test-file/test.txt.
If the file test-file/test.txt is not available in test-bucket, OBS sends a request to https://testexample.com/test-file/test.txt on the origin server to fetch the file.
After the origin server returns the requested file to OBS, OBS saves the test.txt file to the test-file folder in test-bucket and returns that file to the requester.

Replacing the Requested File Prefix During Retrieval

In certain service scenarios, the file path in an OBS bucket differs from that on the origin server. To ensure the correct file is returned, you need to configure a replacement prefix in the back-to-source rule to map the file path in the bucket to that on the origin server.

Step 1 Configure a back-to-source rule for bucket test-bucket.

In the navigation pane, choose Data Management > Back to Source. The back-to-source rule list is displayed.
Click Create.

Configure the parameters as described in the following table. Keep the default settings for other parameters.

Click to enlarge

**Table 8** Parameters
Parameter	Value Example	Description
File Name Prefix	test-file/	Prefix of the file to be retrieved. In this example, set this parameter to test-file/, which indicates that the rule is triggered when the requested file is not found in the test-file folder in the bucket.
Prefix and Suffix	Prefix and Suffix: Replace prefix New String: file/	Whether OBS adds a prefix or suffix to the name of the object requested by the client when OBS requests data from the origin server. Replace prefix: OBS uses the characters specified here to replace the file name prefix defined in the back-to-source condition. However, the object returned to the client keeps its original name and does not include the replacement prefix. Example: If the file name prefix configured in the back-to-source condition is test-file/ and the replacement prefix is file/, then when a client requests test-file/test.txt and triggers the back-to-source rule, OBS changes the object name requested from the origin server to file/test.txt. This object is then downloaded to OBS, saved as test-file/test.txt, and returned to the client.
Active Site	Protocol: https Source domain name: testexample.com Static path: Leave it blank.	The active site of the origin URL. Format: http://Source domain name/Static path or https://Source domain name/Static path Set the protocol to http or https. Set the source domain name to the domain name of the origin server. Set the static path to the directory where the target file is located on the origin server. Example: If the static path is set to 123/, the target file is in the 123/ directory on the origin server.

Click OK.

Step 2 Verify the rule.

Access https://test-bucket.obs.cn-north-4.myhuaweicloud.com/test-file/test.txt.
If the file test-file/test.txt is not available in test-bucket, OBS sends a request to https://testexample.com/file/test.txt on the origin server to fetch the file.
After the origin server returns the requested file to OBS, OBS saves the test.txt file to the test-file folder in test-bucket and returns that file to the requester.

Retrieving Data from Different Origin Servers Based on the Requested File Directory

In certain multi-service scenarios, OBS needs to access different origin servers to obtain the required resources. In addition, the retrieved service resources need to be integrated and categorized for easier service migration.

Example: Origin server A (https://testexample01.com/) is used to store text files, while origin server B (https://testexample02.com/) is used to store image files.

If you want to achieve the following behavior:

If a client requests a test-txt/test.txt file not found in test-bucket in the CN North-Beijing4 region, the file is obtained from txt/ of origin server A.
If a client requests a test-image/myimage.jpg file that does not exist in test-bucket in the CN North-Beijing4 region, the file is obtained from image/ on origin server B.

Then, you need to configure back-to-source rules as follows:

Step 1 Configure back-to-source rules for bucket test-bucket.

You need to configure two back-to-source rules for bucket test-bucket. In one rule, set the origin server to origin server A. In the other rule, set the origin server to origin server B.

In the navigation pane, choose Data Management > Back to Source. The back-to-source rule list is displayed.
Click Create.

Configure the parameters as described in the following table. Keep the default settings for other parameters.

Click to enlarge

**Table 9** Parameters
Parameter	Value Example	Description
File Name Prefix	test-txt/	Prefix of the file to be retrieved. In this example, set this parameter to test-txt/, which indicates that the rule is triggered when the requested file is not found in the test-txt folder in the bucket.
Prefix and Suffix	Prefix and Suffix: Replace prefix New String: txt/	Whether OBS adds a prefix or suffix to the name of the object requested by the client when OBS requests data from the origin server. Replace prefix: OBS uses the characters specified here to replace the file name prefix defined in the back-to-source condition. However, the object returned to the client keeps its original name and does not include the replacement prefix. Example: If the file name prefix configured in the back-to-source condition is test-txt/ and the replacement prefix is file/, then when a client requests test-txt/test.txt and triggers the back-to-source rule, OBS changes the object name requested from the origin server to txt/test.txt. This object is then downloaded to OBS, saved as test-txt/test.txt, and returned to the client.
Active Site	Protocol: https Source domain name: testexample01.com Static path: Leave it blank.	The active site of the origin URL. Format: http://Source domain name/Static path or https://Source domain name/Static path Set the protocol to http or https. Set the source domain name to the domain name of the origin server. Set the static path to the directory where the target file is located on the origin server. Example: If the static path is set to 123/, the target file is in the 123/ directory on the origin server.

Click OK.

Repeat 1 to 4 to configure the second back-to-source rule.

Click to enlarge

**Table 10** Parameters
Parameter	Value Example	Description
File Name Prefix	test-image/	Prefix of the file to be retrieved. In this example, set this parameter to test-image/, which indicates that the rule is triggered when the requested file is not found in the test-image folder in the bucket.
Prefix and Suffix	Prefix and Suffix: Replace prefix New String: image/	Whether OBS adds a prefix or suffix to the name of the object requested by the client when OBS requests data from the origin server. Replace prefix: OBS uses the characters specified here to replace the file name prefix defined in the back-to-source condition. However, the object returned to the client keeps its original name and does not include the replacement prefix. Example: If the file name prefix configured in the back-to-source condition is test-image/ and the replacement prefix is image/, then when a client requests test-image/myimage.jpg and triggers the back-to-source rule, OBS changes the object name requested from the origin server to image/myimage.jpg. This object is then downloaded to OBS, saved as test-image/myimage.jpg, and returned to the client.
Active Site	Protocol: https Source domain name: testexample02.com Static path: Leave it blank.	The active site of the origin URL. Format: http://Source domain name/Static path or https://Source domain name/Static path Set the protocol to http or https. Set the source domain name to the domain name of the origin server. Set the static path to the directory where the target file is located on the origin server. Example: If the static path is set to 123/, the target file is in the 123/ directory on the origin server.

Step 2 Verify the rule.

Access the text file https://test-bucket.obs.cn-north-4.myhuaweicloud.com/test-txt/test.txt.
1. If the file test-txt/test.txt is not available in test-bucket, OBS sends a request to https://testexample01.com/txt/test.txt on the origin server to fetch the file.
2. After the origin server returns the requested file to OBS, OBS saves the test.txt file to the test-txt folder in test-bucket and returns that file to the requester.
Access the image file https://test-bucket.obs.cn-north-4.myhuaweicloud.com/test-image/myimage.jpg.
1. If the test-image/myimage.jpg file is not found in test-bucket, OBS sends a request to the origin server https://testexample02.com/image/myimage.jpg to obtain the file.
2. After the origin server returns the target file to OBS, OBS saves the myimage.jpg file to the test-image folder in test-bucket and returns the target file to the requester.

Retrieving Data from the Origin Server and Passing Specified Client Parameters to the Origin Server

If you want to achieve the following behavior:

If a client requests a test-file/test.txt file not found in test-bucket in the CN North-Beijing4 region, the file is obtained from file/ on the origin server (https://testexample.com/).
HTTP headers (header1, header2, and header3) included in the request URL can be passed to the origin server.
HTTP headers (header4 and header5) included in the request URL cannot be passed to the origin server.

Then, you need to configure the back-to-source rule as follows:

Step 1 Configure back-to-source rules for bucket test-bucket.

In the navigation pane, choose Data Management > Back to Source. The back-to-source rule list is displayed.
Click Create.

Configure the parameters as described in the following table. Keep the default settings for other parameters.

Click to enlarge

**Table 11** Parameters
Parameter	Value Example	Description
File Name Prefix	test-file/	Prefix of the file to be retrieved. In this example, set this parameter to test-file/, which indicates that the rule is triggered when the requested file is not found in the test-file folder in the bucket.
Prefix and Suffix	Prefix and Suffix: Replace prefix New String: file/	Whether OBS adds a prefix or suffix to the name of the object requested by the client when OBS requests data from the origin server. Replace prefix: OBS uses the characters specified here to replace the file name prefix defined in the back-to-source condition. However, the object returned to the client keeps its original name and does not include the replacement prefix. Example: If the file name prefix configured in the back-to-source condition is test-file/ and the replacement prefix is file/, then when a client requests test-file/test.txt and triggers the back-to-source rule, OBS changes the object name requested from the origin server to file/test.txt. This object is then downloaded to OBS, saved as test-file/test.txt, and returned to the client.
Active Site	Protocol: https Source domain name: testexample.com Static path: Leave it blank.	The active site of the origin URL. Format: http://Source domain name/Static path or https://Source domain name/Static path Set the protocol to http or https. Set the source domain name to the domain name of the origin server. Set the static path to the directory where the target file is located on the origin server. Example: If the static path is set to 123/, the target file is in the 123/ directory on the origin server.
HTTP Header Pass Rule	Pass specified parameters: header1 header2 header3 Do not pass specified parameters: header4 header5	You can specify the HTTP header parameters that can be passed to the origin server when a request sent to OBS triggers the back-to-source by mirroring rule. Pass specified parameters: Set the HTTP header parameters that can be passed. Do not pass specified parameters: Set the HTTP header parameters that cannot be passed. If a client's request includes the specified headers, OBS will not pass them to the origin server. If a header is specified for both the pass and do-not-pass categories, it is deemed as a do-not-pass parameter.

Click OK.

Step 2 Verify the rule.

Access https://test-bucket.obs.cn-north-4.myhuaweicloud.com/test-file/test.txt.

The request headers are listed below.

GET /ObjectName HTTP/1.1 
Host: test-bucket.obs.cn-north-4.myhuaweicloud.com 
header1:aaa
header2:bbb
header3:ccc
header4:ddd
header5:eee

If the file test-file/test.txt is not available in test-bucket, OBS sends a request to https://testexample.com/file/test.txt on the origin server to fetch the file.

According to the HTTP header pass rule configured in the back-to-source rule, some of the request headers sent by OBS to the origin server are listed below.
```
GET /ObjectName HTTP/1.1 
Host: source.com 
header1:aaa
header2:bbb
header3:ccc
```
After the origin server returns the requested file to OBS, OBS saves the test.txt file to the test-file folder in test-bucket and returns that file to the requester.

Obtaining Files from the Redirected Address After Origin Server Polling

If you want to achieve the following behavior:

If a client requests a test-file/test.txt file not found in test-bucket in the CN North-Beijing4 region, the file is obtained from file/ on origin server A (https://testexample01.com/).
If the file does not exist in file/ on origin server A, the file is obtained from file/ on origin server B (https://testexample02.com/).
If the file is not found on origin server B either, OBS requests the file from the redirected address if a redirection policy is configured for origin servers A and B.

Then, you need to configure the back-to-source rule as follows:

Step 1 Configure back-to-source rules for bucket test-bucket.

In the navigation pane, choose Data Management > Back to Source. The back-to-source rule list is displayed.
Click Create.

Configure the parameters as described in the following table. Keep the default settings for other parameters.

Click to enlarge

**Table 12** Parameters
Parameter	Value Example	Description
File Name Prefix	test-file/	Prefix of the file to be retrieved. In this example, set this parameter to test-file/, which indicates that the rule is triggered when the requested file is not found in the test-file folder in the bucket.
Prefix and Suffix	Prefix and Suffix: Replace prefix New String: file/	Whether OBS adds a prefix or suffix to the name of the object requested by the client when OBS requests data from the origin server. Replace prefix: OBS uses the characters specified here to replace the file name prefix defined in the back-to-source condition. However, the object returned to the client keeps its original name and does not include the replacement prefix. Example: If the file name prefix configured in the back-to-source condition is test-file/ and the replacement prefix is file/, then when a client requests test-file/test.txt and triggers the back-to-source rule, OBS changes the object name requested from the origin server to file/test.txt. This object is then downloaded to OBS, saved as test-file/test.txt, and returned to the client.
Active Site	Active site 1 Protocol: https Source domain name: testexample01.com Static path: Leave it blank. Active site 2 Protocol: https Source domain name: testexample02.com Static path: Leave it blank.	The active site of the origin URL. Format: http://Source domain name/Static path or https://Source domain name/Static path Set the protocol to http or https. Set the source domain name to the domain name of the origin server. Set the static path to the directory where the target file is located on the origin server. Example: If the static path is set to 123/, the target file is in the 123/ directory on the origin server.
Redirect requests to the returned location	Enabled	After this option is enabled, if redirection is configured for the origin server, the resources will be requested from the location included in the 3xx response returned by the origin server and be saved to OBS. Each request supports up to 10 redirects.

Click OK.

Step 2 Verify the rule.

Access https://test-bucket.obs.cn-north-4.myhuaweicloud.com/test-file/test.txt.
If the file test-file/test.txt is not available in test-bucket, OBS sends a request to https://testexample.com01/file/test.txt on origin server A to fetch the file.
- If origin server A does not have the test.txt file and returns a 404 error, OBS sends a request to https://testexample.com02/file/test.txt on origin server B to fetch the file.
- If origin server A or B returns a 3xx status code, OBS sends a request to the redirection address returned by the origin server. After obtaining the file, OBS saves the file as test-file/test.txt to test-bucket and returns the file to the client.

References

Creating an Agency for Back to Source

On OBS Console, click View Agencies in the Create Back-to-Source Rule dialog box to navigate to the Agencies page on the IAM console.

Click Create Agency. On the displayed page, configure the parameters related to the agency for back to source. Keep the default settings for other parameters.

**Table 13** Parameters
Parameter	Description
Agency Name	The name of the agency to be created The agency name cannot be left blank.
Agency Type	Selecting Cloud service grants permissions to an account or cloud service to operate resources within the account. In this example, you need to create an agency for back to source, so select Cloud service here.
Cloud Service	Select the cloud service to which you want to grant permissions to operate resources in your account. This parameter is mandatory when you set Agency Type to Cloud service. In this example, select Object Storage Service (OBS).

Click OK. In the Agency Created dialog box that is displayed, click Cancel. If the newly created agency appears in the agency list, the agency is created successfully.

Create a custom policy that grants OBS permissions to the agency. If a policy that meets the requirements already exists, skip this step.

In the navigation pane of the IAM console, choose Permissions > Policies/Roles.
On the Policies/Roles page, click Create Custom Policy in the upper right corner.

On the Create Custom Policy page, configure the parameters as described below. Keep the default settings for other parameters.

Policy Name: Enter a name that is easy to remember.
Select service: Select Object Storage Service (OBS).

Actions: Select the actions listed in the table below.

**Table 14** OBS permissions required by the agency
Action	Description
obs:object:GetObject	Obtains object content and metadata.
obs:object:PutObject	Uploads objects using PUT or POST, copies objects, appends data to objects, initiates multipart uploads, uploads parts, assembles parts, modifies objects, truncates objects, and renames objects.
obs:object:AbortMultipartUpload	Cancels a multipart upload.
obs:bucket:ListBucket	Lists objects in a bucket.

For Resources, select All.

Click OK. If the created custom policy appears in the list, it was created successfully.

Apply the created custom policy to the agency.
1. In the navigation pane of the IAM console, choose Agencies.
2. Click Authorize in the Operation column of the agency created in 2.
3. On the Select Policy/Role page, select the custom policy created in 4 and click Next.
4. On the Select Scope page, select All resources and click OK.
5. In the Information dialog box that is displayed, click OK.
6. On the Finish page, click Finish.

FAQ

Parent Topic: Data Management

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Using Mirroring-Based Back to Source to Retrieve Data

Process of Back to Source by Mirroring

Back-to-Source Rules

Constraints

Important Notes

Creating a Back-to-Source by Mirroring Rule

Using OBS Console

Using APIs

Replicating Back-to-Source by Mirroring Rules

Back to Source by Mirroring Examples

Obtaining Missing Files from a Specified Website

Replacing the Requested File Prefix During Retrieval

Retrieving Data from Different Origin Servers Based on the Requested File Directory

Retrieving Data from the Origin Server and Passing Specified Client Parameters to the Origin Server

Obtaining Files from the Redirected Address After Origin Server Polling

References

Creating an Agency for Back to Source

FAQ

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