Apache MPM

Apache MPM
Name	Apache MPM
Developer	Apache Software Foundation
Operating system	Cross-platform
License	Apache License 2.0

Apache MPM

Apache MPM is a component of the Apache HTTP Server that controls the server's process and thread model for handling HTTP requests. It mediates concurrency, resource allocation, and lifecycle behavior in interaction with the kernel and system libraries, influencing throughput, latency, and memory usage. Decisions about MPM selection affect deployment on platforms like Linux, FreeBSD, Microsoft Windows, and macOS and intersect with projects such as OpenSSL, mod_ssl, PHP, and Tomcat.

Overview

MPMs operate within the Apache Software Foundation project, integrating with modules like mod_wsgi and mod_proxy to coordinate request handling. They determine how the server forks processes or spawns threads, which in turn impacts integration with runtimes such as Perl, Ruby, Python, and Java application servers including GlassFish and Jetty. MPM behavior interacts with system facilities like epoll, kqueue, and IOCP and with container runtimes from Docker and orchestration by Kubernetes.

Multiprocessing Modules (MPMs) in Apache HTTP Server

An MPM encapsulates runtime strategies used by releases of Apache HTTP Server maintained by contributors from the Apache HTTP Server Project and downstream distributions such as Debian, Red Hat Enterprise Linux, Ubuntu, CentOS, and Alpine Linux. MPMs are implemented as loadable modules or compile-time options and coordinate with event notification frameworks like libevent and libuv. Governance, issue tracking, and patches often appear in repositories on platforms such as GitHub and GitLab, while large deployments are influenced by vendors including IBM, Oracle, Google, and Microsoft.

Common MPM Types (prefork, worker, event, hybrid)

Common MPMs include one-process-per-request models and threaded hybrid models. The prefork-style approach resembles practices used in legacy deployments at organizations like NASA and CERN, while threaded models are competitive in enterprise stacks at Facebook and Netflix where concurrency demands mirror those in Apache Tomcat and Nginx. Event-driven MPMs employ readiness notification similar to techniques in Nginx and Lighttpd, leveraging APIs created for Linux Kernel features and influenced by research appearing in conferences like USENIX and ACM SIGCOMM.

Configuration and Tuning

Tuning an MPM requires adjusting directives often exposed by distributions such as Debian GNU/Linux and configuration management tools like Ansible, Puppet, Chef, and SaltStack. Administrators consider limits found in systemd unit files, ulimit settings, and cgroup policies used by systemd and LXC containers. Performance counters gathered by Prometheus, Grafana, and New Relic inform settings for connection backlogs, thread pools, and accept mutex behavior studied in whitepapers by vendors like Red Hat and researchers from MIT and Stanford University.

Performance, Scalability, and Resource Usage

MPM choice shapes metrics tracked in benchmarks such as those by SPEC, Phoronix Test Suite, and reports from companies like Mozilla and LinkedIn. Memory footprint comparisons reference allocators like jemalloc and tcmalloc and profiling tools such as valgrind and perf. Scalability under load ties to OS scheduling in System V, POSIX, and integration with load balancers from HAProxy and F5 Networks or CDN providers including Cloudflare and Akamai.

Compatibility, Security, and Stability Considerations

Selecting an MPM affects compatibility with language bindings and security modules like mod_security and cryptographic stacks such as OpenSSL and LibreSSL. Vulnerability disclosures coordinated with the Common Vulnerabilities and Exposures system and advisories from organizations including US-CERT and CERT/CC may recommend configuration changes or MPM upgrades. Stability testing uses CI systems like Jenkins and Travis CI and formal verification techniques referenced in publications from Carnegie Mellon University and ETH Zurich.

Implementation History and Evolution

The MPM architecture evolved across versions of the Apache HTTP Server project and in response to contributions at events such as ApacheCon and papers presented at USENIX ATC; early designs were influenced by server models in NCSA HTTPd and by threaded web server research from institutions such as University of California, Berkeley and University of Illinois Urbana–Champaign. Maintenance has involved committers affiliated with companies like Red Hat, Canonical, IBM, and Intel and has been tracked through mailing lists maintained by the Apache Software Foundation and issue trackers used by Linux distributions including SUSE and Fedora.

Category:Web server software