Share Data Across Requests Served by OpenResty

This tutorial will demonstrate how to share data across different HTTP requests served by an OpenResty application.

cd ~/
mkdir data-share
cd data-share/
mkdir conf logs lua
tree

We first prepare our test application’s directory tree.

The easist way to share data among requests is to use a custom Lua module named lua/my-module.lua.

We make the following edits:

1. We add a global counter variable on the top-level of this module.
2. Then we always increment this counter in a module function named main.
3. and return the new counter value.

local _M = {}

local counter = 0

function _M.main()
    counter = counter + 1
    return counter
end

return _M

Now let’s quickly craft the nginx configuration file, conf/nginx.conf.

We make the following edits:

1. Here we specify where to look for our Lua module.
2. Then define a server listening on the local 8080 port.
3. And a root location with content_by_lua_block.
4. Load up our Lua module and invoke its main function.
5. And output the returned counter value as the response body.

worker_processes 1;

events {
    worker_connections 1024;
}

http {
    lua_package_path "$prefix/lua/?.lua;;";

    server {
        listen 8080;

        location / {
            default_type text/plain;
            content_by_lua_block {
                local mod = require "my-module"
                local cnt = mod.main()
                ngx.say("counter = ", cnt)
            }
        }
    }
}

Let’s check the directory tree again.

tree .

Looking good.

Start this OpenResty application without sudo.

openresty -p $PWD/

Here we only enable a single worker process.

ps aux|grep nginx|grep -v tmp

Send an HTTP request to the server using curl:

curl 'http://127.0.0.1:8080/'

The counter value is 1.

Again.

Cool, it’s 2 now.

It will only keep growing.

This counter Lua variable is shared because it is an up value (upvalue) of the Lua module function.

cat lua/my-module.lua

And the Lua module is cached and shared in the global LuaJIT virtual machine.

resty -Ilua -e 'require "my-module" print(package.loaded["my-module"].main)'

See? We can also access the cached Lua module directly.

The package.loaded global table holds all the loaded Lua modules.

resty -e 'for k, v in pairs(package.loaded) do print(k) end'

Here, most of the modules are those standard ones.

There is a limitation, however. Such data cannot be shared among different nginx worker processes.

It’s not a problem here because only 1 worker process is configured.

But usually we would utilize multiple CPU cores by enabling multiple worker processes.

worker_processes 4;

Then each worker process would have its own counter.

Let’s test the configuration and then reload the server.

openresty -p $PWD/ -t
kill -HUP `cat logs/nginx.pid`

Now we should have 4 worker processes.

ps aux|grep nginx|grep -v tmp

So this Lua module data sharing approach is mostly useful for Lua level data caching.

The resty.lrucache Lua module shipped with OpenResty is designed for such use cases.

restydoc resty.lrucache

We will cover this module in a future tutorial.

If we want to share data across all the worker processes, we should use the Lua shared memory dictionaries instead.

restydoc -s lua_shared_dict
restydoc -s ngx.shared.DICT

We will cover this topic in another dedicated tutorial.

That’s all I’d like to cover today. Happy hacking!

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About This Article and Associated Video

This article and its associated video are both generated automatically from a simple screenplay file.

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About The Author

Yichun Zhang is the creator of the OpenResty^® open source project. He is also the founder and CEO of the OpenResty Inc. company. He contributed a dozen open source Nginx 3rd-party modules, quite some Nginx and LuaJIT core patches, and designed the OpenResty XRay platform.

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