AS3 Best Practice


AS3 is a declarative API that uses JSON key-value pairs to describe a BIG-IP configuration. From virtual IP to virtual server, to the members, pools, and nodes required, AS3 provides a simple, readable format in which to describe a configuration. Once you've got the configuration, all that's needed is to POST it to the BIG-IP, where the AS3 extension will happily accept it and execute the commands necessary to turn it into a fully functional, deployed BIG-IP configuration.

If you are new to AS3, start reading the following references:

This article describes some considerations in order to efficiently deploy the AS3 configurations.



In the TMOS space, the services that AS3 provides are processed by a daemon named 'restnoded'. It relies on the existing BIG-IP framework for deploying declarations. The framework consists of httpd, restjavad and icrd_child as depicted below (the numbers in parenthesis are listening TCP port numbers).

These processes are also used by other services. For example, restjavad is a gateway for all the iControl REST requests, and is used by a number of services on BIG-IP and BIG-IQ. When an interaction between any of the processes fails, AS3 operation fails. The failures stem from lack of resources, timeouts, data exceeding predefined thresholds, resource contention among the services, and more. In order to complete AS3 operations successfully, it is advised to follow the Best Practice outlined below.


Best Practice

Your single source of truth is your declaration

Refrain from overwriting the AS3-deployed BIG-IP configurations by the other means such as TMSH, GUI or iControl REST calls.

Since you started to use the AS3 declarative model, the source of truth for your device's configurations is in your declaration, not the BIG-IP configuration files. Although AS3 tries to weigh BIG-IP locally stored configurations as much as it can do, discrepancy between the declaration and the current configuration on BIG-IP may cause the AS3 to perform less efficiently or error unexpectedly. When you wish to change a section of a tenant (e.g., pool name change), modify the declaration and submit it.

Keep the number of applications in one tenant to a minimum

AS3 processes each tenant separately. Having too many applications (virtual servers) in a single tenant (partition) results in a lengthy poll when determining the current configuration. In extreme cases (thousands of virtuals), the action may time out. When you want to deploy a thousand or more applications on a single device, consider chunking the work for AS3 by spreading the applications across multiple tenants (say, 100 applications per tenant).

AS3 tenant access behavior behaves as BIG-IP partition behavior. A non-Common partition virtual cannot gain access to another partition's pool, and in the same way, an AS3 application does not have access to a pool or profile in another tenant. In order to share configuration across tenants, AS3 allows configuration of the "Shared" application within the "Common" tenant. AS3 avoids race conditions while configuring /Common/Shared by processing additions first and deletions last, as shown below. This dual process may cause some additional delay in declaration handling.

Overwrite rather than patching (POSTing is a more efficient practice than PATCHing)

AS3 is a stateless machine and is idempotent. It polls BIG-IP for its full configuration, performs a current-vs-desired state comparison, and generates an optimal set of REST calls to fill the differences. When the initial state of BIG-IP is blank, the poll time is negligible. This is why initial configuration with AS3 is often quicker than subsequent changes, especially when the tenant contains a large number of applications.

AS3 provides the means to partially modify using PATCH (see AS3 API Methods Details), but do not expect PATCH changes to be performant. AS3 processes each PATCH by (1) performing a GET to obtain the last declaration, (2) patching that declaration, and (3) POSTing the entire declaration to itself. A PATCH of one pool member is therefore slower than a POST of your entire tenant configuration. If you decide to use PATCH, make sure that the tenant configuration is a manageable size. 

Note: Using PATCH to make a surgical change is convenient, but using PATCH over POST breaks the declarative model. Your declaration should be your single source of truth. If you include PATCH, the source of truth becomes "POST this file, then apply one or more PATCH declarations."

Get the latest version

AS3 is evolving rapidly with new features that customers have been wishing for along with fixes for known issues.

Visit the AS3 section of the F5 Networks GithubIssues section shows what features and fixes have been incorporated.

For BIG-IQ, check K54909607: BIG-IQ Centralized Management compatibility with F5 Application Services 3 Extension and F5 Declarative Onboarding for compatibilities with BIG-IQ versions before installation.

Use administrator

Use a user with the administrator role when you submit your declaration to a target BIG-IP device. Your may find your role insufficient to manipulate BIG-IP objects that are included in your declaration. Even one authorized item will cause the entire operation to fail and role back. 

See the following articles for more on BIG-IP user and role.

Use Basic Authentication for a large declaration

You can choose either Basic Authentication (HTTP Authorization header) or Token-Based Authentication (F5 proprietary X-F5-Auth-Token) for accessing BIG-IP. While the Basic Authentication can be used any time, a token obtained for the Token-Based Authentication expires after 1,200 seconds (20 minutes). While AS3 does re-request a new token upon expiry, it requires time to perform the operation, which may cause AS3 to slow down. Also, the number of tokens for a user is limited to 100 (since 13.1), hence if you happen to have other iControl REST players (such as BIG-IQ or your custom iControl REST scripts) using the Token-Based Authentication for the same user, AS3 may not be able to obtain the next token, and your request will fail.

See the following articles for more on the Token-Based Authentication.

Choose the best window for deployment

AS3 (restnoded daemon) is a Control Plane process. It competes against other Control Plane processes such as monpd and iRules LX (node.js) for CPU/memory resources. AS3 uses the iControl REST framework for manipulating the BIG-IP resources. This implies that its operation is impacted by any processes that use httpd (e.g., GUI), restjavad, icrd_child and mcpd. If you have resource-hungry processes that run periodically (e.g., avrd), you may want to run your AS3 declaration during some other time window.

See the following K articles for a list of processes


If you experience issues such as timeout on restjavad, it is possible that your AS3 operation had resource issues. After reviewing the Best Practice above but still unable to alleviate the problem, you may be able to temporarily fix it by applying the following tactics. 

Increase the restjavad memory allocation

The memory size of restjavad can be increased by the following tmsh sys db commands

tmsh modify sys db provision.extramb value <value>

tmsh modify sys db restjavad.useextramb value true

The provision.extramb db key changes the maximum Java heap memory to (192 + <value> * 8 / 10) MB. The default value is 0. After changing the memory size, you need to restart restjavad.

tmsh restart sys service restjavad

See the following article for more on the memory allocation:

Increase a number of icrd_child processes

restjavad spawns a number of icrd_child processes depending on the load. The maximum number of icrd_child processes can be configured from /etc/icrd.conf. Please consult F5 Support for details.

See the following article for more on the icrd_child process verbosity:

Decrease the verbosity levels of restjavad and icrd_child

Writing log messages to the file system is not exactly free of charge. Writing unnecessarily large amount of messages to files would increase the I/O wait, hence results in slowness of processes. If you have changed the verbosity levels of restjavad and/or icrd_child, consider rolling back the default levels.

See the following article for methods to change verbosity level:

Updated May 02, 2023
Version 3.0

Was this article helpful?


  • Are there any limitations when using one partition per application when we reach a high number of partitions ?

  • well described, thank you. And interesting news about basic authentication and token reusing limitations in v13.x