Caching
Caching is a technique used to store frequently accessed data in memory, enabling faster retrieval and reducing the load on underlying systems. In Stalwart, caching is utilized for various critical operations, including storing email account data, security access tokens, and DNS query results. By caching this information, the server enhances performance, reduces latency, and minimizes resource usage.
Stalwart employs an efficient caching algorithm with an advanced eviction policy. The eviction mechanism is a modified version of the Clock-PRO algorithm, which closely resembles the later-published S3-FIFO algorithm. This algorithm is designed to be “scan-resistant,” meaning it avoids cache eviction caused by temporary spikes in access patterns. It provides high cache hit rates, outperforming traditional Least Recently Used (LRU) eviction policies and delivering results comparable to other state-of-the-art algorithms, such as W-TinyLFU. This advanced approach ensures that cached data remains highly relevant, contributing to the server's overall efficiency.
Memory Usage
The memory usage of the cache is an important consideration, especially in systems with varying hardware configurations and user loads. For systems with ample memory and a high number of users, it is recommended to increase the default cache sizes. Larger cache sizes allow the server to store more data in memory, resulting in fewer evictions and higher hit rates, which improves performance in busy environments.
Conversely, systems with limited memory may need to reduce the default cache sizes to avoid excessive memory consumption. Smaller cache sizes help conserve system resources but may slightly reduce cache efficiency. Administrators should monitor the server’s memory usage and adjust cache sizes to strike a balance between performance and resource availability.
Configuration
Stalwart provides a variety of cache options that can be configured to optimize performance and memory usage. Each cache is configured under the cache.NAME.size parameter, where NAME is the name of the cache as defined in the system. The size is specified in bytes and controls the maximum memory allocated for each cache.
Email Cache
The Email Cache stores email account data, including message UIDs, flags, and mailbox information. By caching account data, the system ensures fast and efficient interactions with the email server. It is configured under the cache.email.size setting and has a default size of 50 MB.
[cache]
message.size = 52428800
Calendar Cache
The Calendar Cache stores calendar-related data, such as event start/end times and timezone information. This cache improves the performance of calendar operations, including event retrieval and scheduling. It is configured under the cache.events.size setting and has a default size of 10 MB.
[cache]
events.size = 10485760
Contacts Cache
The Contacts Cache stores contact-related data, including contact metadata and other relevant information. This cache enhances the performance of contact-related operations, such as searching and retrieving contact details. It is configured under the cache.contacts.size setting and has a default size of 10 MB.
[cache]
contacts.size = 10485760
File Storage Cache
The File Storage Cache stores metadata related to file storage operations, such as file paths and access permissions. This cache improves the performance of file-related operations, such as listing files and checking permissions. It is configured under the cache.files.size setting and has a default size of 10 MB.
[cache]
files.size = 10485760
Security Cache
Access Tokens
The Access Token Cache is used to store security access tokens required for authentication purposes. Caching access tokens reduces the need for frequent revalidation, ensuring faster and more efficient authentication processes. It is configured under the cache.access-token.size setting and has a default size of 10 MB.
[cache]
access-token.size = 10485760
HTTP Authentication
The HTTP Authentication Cache stores temporary data related to HTTP-based authentication. By caching this data, the system speeds up repeated authentication attempts, reducing the overhead for frequent HTTP requests. It is configured under the cache.http-auth.size setting and has a default size of 1 MB.
[cache]
http-auth.size = 1048576
Permissions
The Permissions Cache is used to store role and permission mappings for user accounts. This cache allows the system to quickly verify access rights without repeatedly querying the database. It is configured under the cache.permission.size setting and has a default size of 5 MB.
[cache]
permission.size = 5242880
Bayesian Filter Cache
The Bayesian Filter Cache stores token data and associated weights for the spam filter’s Bayesian classifier. Caching this data allows the spam filter to make quick and accurate decisions without frequently accessing the database. It is configured under the cache.bayes.size setting and has a default size of 10 MB.
[cache]
bayes.size = 10485760
DNS Cache
TXT Records
The TXT DNS Cache stores the results of DNS TXT record lookups, which are commonly used for SPF, DKIM, and DMARC verifications. By caching these results, the server reduces the latency of DNS queries. It is configured under the cache.dns.txt.size setting and has a default size of 5 MB.
[cache]
dns.txt.size = 5242880
MX Records
The MX DNS Cache stores the results of DNS MX record lookups, which are essential for email routing and delivery. Caching these results improves the speed of email processing. It is configured under the cache.dns.mx.size setting and has a default size of 5 MB.
[cache]
dns.mx.size = 5242880
PTR Records
The PTR DNS Cache stores the results of reverse DNS (PTR) record lookups, which resolve IP addresses to hostnames. This cache enhances the efficiency of reverse DNS queries. It is configured under the cache.dns.ptr.size setting and has a default size of 1 MB.
[cache]
dns.ptr.size = 1048576
IPv4 Records
The IPv4 DNS Cache stores the results of IPv4 address queries. Caching these results reduces the frequency of DNS lookups, improving overall performance. It is configured under the cache.dns.ipv4.size setting and has a default size of 5 MB.
[cache]
dns.ipv4.size = 5242880
IPv6 Records
The IPv6 DNS Cache stores the results of IPv6 address queries. Like the IPv4 DNS Cache, it reduces the need for repeated DNS lookups and improves efficiency. It is configured under the cache.dns.ipv6.size setting and has a default size of 5 MB.
[cache]
dns.ipv6.size = 5242880
TLSA Records
The TLSA DNS Cache stores the results of TLSA record lookups, which are used for DANE authentication. By caching these records, the server ensures faster processing of secure email communication. It is configured under the cache.dns.tlsa.size setting and has a default size of 1 MB.
[cache]
dns.tlsa.size = 1048576
MTA-STS Records
The MTA-STS Cache stores Mail Transfer Agent Strict Transport Security (MTA-STS) policies, which are used to enforce secure email delivery. This cache reduces the need for frequent policy retrievals. It is configured under the cache.dns.mta-sts.size setting and has a default size of 1 MB.
[cache]
dns.mta-sts.size = 1048576
RBL Records
The RBL DNS Cache stores the results of Real-Time Blocklist (RBL) queries, which are used to identify and block potentially malicious IP addresses. This cache minimizes the latency of RBL lookups. It is configured under the cache.dns.rbl.size setting and has a default size of 5 MB.
[cache]
dns.rbl.size = 5242880