eval
The `eval` directive allows capturing the response of a subrequest into a variable for later use in NGINX configuration.
Description
The eval directive in NGINX is a powerful feature provided by the ngx_eval module that enables the evaluation of HTTP subrequests, allowing the response from these subrequests to be stored in a variable. This is particularly useful for leveraging the output of upstream services, databases, or other resources within the primary request handling flow. The directive can accept one or more blocks of subrequest handling instructions, each designed to retrieve specific data based on provided directives. The responses captured can then be manipulated or analyzed via standard NGINX variable syntax.
When using eval, the contained block can include commands such as drizzle_query or postgres_query, and options to configure the evaluation context, including how to handle content types and whether the content should be kept in memory. The buffer size for responses can also be specified, affecting how much data can be captured before it is truncated, which is crucial for processing larger payloads from the subrequest responses. Additionally, the use of eval can be paired with other processing directives like if statements to create dynamic request handling scenarios based on the retrieved data.
It is essential to remember that although the eval directive is flexible and powerful, it can introduce complexities in error handling and debugging, given the asynchronous nature of subrequest processing. Developers are advised to employ careful configuration management and testing strategies to ensure that subrequest evaluations perform as expected.
Config Example
location = /data {
eval $response {
drizzle_query "SELECT * FROM users";
drizzle_pass my_mysql_backend;
}
if ($response ~ 'success') {
return 200;
}
return 404;
}Ensure that the context where eval is applied supports the subrequest operations being called.
The buffer size must be configured properly to avoid truncation of large responses.
Improper handling of content types in eval can lead to incorrect parsing of the response.