Apache Considerations (UNIX®/Linux/macOS)

Apache Process Management and Capacity Planning

Apache provides three process management modules for UNIX® operating systems. In this architecture, the InterSystems Web Gateway modules are directly bound to the Apache worker processes. Therefore, the way Apache is configured to manage its process pool has a direct effect on the Web Gateway.

Apache implements each of its process management models as a Multi-Processing Module (MPM).

Prefork MPM is the traditional multi-process (UNIX®) server architecture. It does not use threads and, as a result, there is no requirement that third-party API modules (DSOs) should be thread-safe. Reference: http://httpd.apache.org/docs/current/mod/prefork.htmlOpens in a new tab.

Worker MPM is the newer hybrid multithread/multi-process server architecture. It does use threads and all third-party API modules (DSOs) used should be thread-safe. Reference: http://httpd.apache.org/docs/current/mod/worker.htmlOpens in a new tab.

Event MPM is designed to allow more requests to be served simultaneously by passing off some processing work to the listener threads, freeing up the worker threads to serve new requests. Reference: http://httpd.apache.org/docs/current/mod/event.htmlOpens in a new tab.

In order to determine which of the server models is in use for an existing installation, call the Apache executable directly but qualified as follows:

httpd -V 

Two further related listings are provided:

• httpd –l List all modules built-in to the server

• httpd –L List all modules and related configuration directives

The Web Gateway DSOs are thread-safe and can be deployed in any server model. A useful guide for Apache tuning can be found here: http://httpd.apache.org/docs/current/misc/perf-tuning.htmlOpens in a new tab.

State-Aware Sessions (Preserve mode 1)

Support for state-aware sessions in a web server that distributes load over multiple worker processes relies on InterProcess Communications (IPC) protocols to manage the routing of requests between individual worker processes. Operating in this web server architecture, the Web Gateway has no control over which worker process handles any particular request.

The Web Gateway uses UNIX® domain sockets for its IPC protocol and the method for supporting state-aware sessions is described below.

As an example, consider a web server installation that distributes its load over 3 worker processes: P1, P2 and P3. Each worker process can potentially start any number of threads (T1, T2 … Tn) according to the web server MPM and configuration in use.

Suppose an application makes a request to mark its session as state-aware (preserve mode 1) and the Web Gateway acknowledges this instruction in process P2. The connection and (security context) to the now private InterSystems IRIS process is hosted by web server worker process P2. All further requests for that user/session must now be processed by worker process P2. However, the Web Gateway has no control over which worker process the web server routes subsequent requests to, so the Web Gateway must establish an IPC channel between P2 and (potentially) any other worker process in the set.

When the Web Gateway marks the connection as state-aware in P2, it starts a listening service in a separate, detached, thread. For log level v2, a message similar to the one shown below is written to the Event Log.

IPC Server 
Process ID: 28457 Listening on Domain Socket: /tmp/csp28457.str

Now, say a further request for the same session is processed by worker process P3. The Web Gateway forwards that request to process P2 via the IPC channel previously established and waits for the response. For log level v2, a message similar to the one shown below is recorded:

Route request over IPC to another web server process
PrivateSession=2; pid_self=28456; ipc_to_pid=28457; 

Of course, if a request for the session happens to be routed by the web server directly to P2, then no further routing is necessary in the Web Gateway environment, since P2 is hosting the session’s private connection.

If the Web Gateway is unable to connect and forward a request to a previously created IPC channel, one of the following messages is recorded depending on the context in which the error was raised:

IPC CLIENT: Error
Cannot connect

Or:

IPC CLIENT: Error
Cannot send request

The most common reason for problems in this area is if Apache has closed (or recycled) a worker process (in the case of the example, P2). Of course, a process can crash (for example, with an access violation/SIGSEGV error) and, in this case, an error message is probably reported in the Apache error log.

Apache also, by default, periodically recycles worker processes.

If you use state-aware sessions, configure Apache such that it doesn’t recycle worker processes by configuring the installation as follows.

• Set the value of MaxConnectionsPerChild to zero

• Set the value of MaxSpareThreads to the same value as MaxRequestWorkers

If it is not possible to prevent Apache periodically recycling processes (perhaps as a result of a malfunctioning module) and state-aware sessions must be used, then an NSD based Gateway configuration can be used. An NSD-based architecture avoids the problems discussed above because it effectively separates the process management of the Web Gateway from the web server. Options for using the Web Gateway’s network service daemon (NSD) are covered in Using the NSD on Microsoft Windows and Using the NSD on UNIX®, Linux, and macOS.