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Async WinBind

The WinBind provider uses libwbclient library for communication with WinBind to satisfy NSS and PAM requests. However this library doesn’t provide an asynchronous interface. We had a choice between creating this interface or use synchronous calls in auxiliary processes running in parallel to the main provider process.

General Approach

There should always be at least one auxiliary process running. This process will receive requests from the main provider process, handle them as they come in and send back responses. The communication protocol used should be DBus as it is used in other providers and therefor doesn’t require any extra dependencies or writing additional code. DBus should also take care of request buffering.

Splitting the load

If the host needs to process a huge amount of NSS and PAM requests in short periods of time, it should be possible to setup more than one auxiliary process to handle them. One should always be available before hand, because starting it just before it’s required adds extra overhead and delay. A maximum (and maybe a minimum too) number of auxiliary process should be configurable along with a threshold expressing when a new process should be created or a spare process killed. The main provider process need to keep track of this and send it’s requests to the least busy auxiliary process.

Implementation steps

  1. Have one auxiliary process started when the provider starts. It will handle all requests.
  2. Add the possibility to have a pre-configured number of processes (maximum=minimum) and split requests between them.
  3. Add the ability to spawn/kill processes based on load.*
  • This needs more thinking: e.g. how long do we keep a spare process alive? how is the threshold going to work?

Update

The main process of the WinBind provider will send requests to spare processes in the pool. These processes will be allocated automatically based on the number of spare processes available at any given time.

Requests from NSS and PAM will be forwarded to spare processes in the pool if there are any available. If not, a new process will be created unless the maximum number of processes has been reached. After the request has been forwarded, the number of available spare processes is checked and a new process is created if there are not enough. Note that the pool is first populated with a minimum number of processes (spare or not) when the WinBind provider starts.

In other words, there will be 3 settings:

Minimum number of worker processes running.

Maximum number of worker processes running.

Minimum number of spare worker processes running.