An IMAP session begins when a user at an IMAP client logs in to the Domino IMAP service. Domino allocates each IMAP session its own session thread from the server's main thread pool. This session thread becomes the sole channel for all communications between the client and the IMAP service. When the session ends, Domino returns the thread to the pool for use by another client.

The session thread communicates directly with the server's IMAP port to receive client input, validate the syntax of received requests, queue requests to the IMAP service, and send responses from the service back to the client. If the IMAP service is slow to respond, the main thread also sends periodic keep-alive messages to the client so that it does not close the connection.

A Domino server can interact with multiple clients simultaneously because it allocates a new thread to service each client session. Clients connect to a port and exchange all input and output through that port. Threads require memory and CPU time. The thread pool contains a limited number of physical threads, but thread use is virtualized so that a single thread works on different tasks. Thus in a fraction of a second, a single thread that is idled by one task as it waits for information, can switch to another task. This allows Domino to maximize processor use and minimize memory.

By avoiding the need to create a new physical thread for each requested connection, Domino makes the best use of available memory. However, a high number of IMAP sessions can place a strain on the server. If clients experience slow response during times of peak usage, consider limiting the number of IMAP sessions.

The internal IMAP thread pool

The Domino IMAP service provides an internal IMAP thread pool that is independent of the thread pool that Domino uses to create client sessions. The default number of available threads is based on the amount of physical memory the server has. The service has a minimum of 50 threads available and a maximum of 400 threads. To ensure that the IMAP service continues to function properly, it's best to use the default thread pool settings and modify these settings only at the direction of a qualified IBM support representative.

The IMAP thread pool consists of three types of worker threads as shown in the following table:

Thread type	Description	Default maximum value
FETCH thread	Accepts validated FETCH commands from the client and transmits them to the Domino mail service	80% of pool total
FETCH response thread	Transmits message data from the Domino mail service to fulfill client FETCH requests	80% of pool total
LOGIN conversion thread	Converts mail files to IMAP format	None

Available threads become active when the main session thread queues a request.

To specify IMAP thread use

1. Make sure you already have a Configuration Settings document for the server(s) to be configured.

2. From the IBM® Lotus® Domino® Administrator, click the Configuration tab and expand the Messaging section.

3. Click Configurations.

4. Select the Configuration Settings document for the mail server or servers you want to restrict mail on, and click Edit Configuration.

5. Click the IMAP - Advanced tab.

6. In the Worker thread pool section, complete the following:

Field	Description
Maximum number of IMAP worker threads	The total number of threads available in the IMAP service's thread pool, including Login conversion threads for upgrading mail files to Domino IMAP format; FETCH threads for transmitting validated client requests to the Domino mail server; and FETCH response threads for transmitting message data from the mail server in response to client FETCH requests.
Maximum number of response threads per FETCH	The number of threads available to transmit message data to fulfill a given FETCH request (default is 4).
Maximum number of FETCH threads allowed	The Number of concurrent threads the IMAP service can use to transmit client requests to FETCH message data to the Domino mail server
Maximum number of FETCH response threads allowed	The number of threads the IMAP service can use to return message data from the Domino mail server in response to FETCH requests received from all active IMAP sessions.

Note These settings apply to Domino Release 7 and later only.

Setting IMAP session limits
Customizing the IMAP service
Creating a Configuration Settings document

Glossary