Session control method and computer-readable storage medium storing computer program

1. A session control method executed by a system in which a first apparatus and a second apparatus are able to communicate with each other via a relay process and a third apparatus and the second apparatus are able to communicate with each other via the relay process, the session control method comprising:

storing control information in which a first connection established on a first apparatus side in relation to the relay process is associated with a second connection established on a second apparatus side in relation to the relay process;

terminating, when a close instruction message at a communication layer higher than the first and second connections is issued from the first apparatus to the second apparatus, the close instruction message on the relay process and updating the control information in response to issuance of the close instruction message; and

enabling, when a switchover notice indicating a switch from the first apparatus to the third apparatus is given and the control information indicates the issuance of the close instruction message, a third connection established on a third apparatus side in relation to the relay process and updating the control information in such a manner that the second connection is associated with the third connection.

2. The session control method according to claim 1 , wherein:

the terminating includes

terminating the close instruction message on the relay process when the close instruction message is issued after a switching start notice indicating start of the switch from the first apparatus to the third apparatus is given but before the switchover notice is given, and

transferring the close instruction message to the second apparatus when the close instruction message is issued before the switching start notice.

3. The session control method of claim 1 , further comprising:

establishing the third connection between the relay process and the third apparatus before the switchover notice is given.

4. The session control method of claim 1 , further comprising:

selecting, based on load on an apparatus running the relay process, the third connection to be established before or after the switchover notice is given.

5. A non-transitory computer-readable storage medium storing a computer program that causes a computer to perform a procedure comprising:

storing control information in which a first connection established on a first apparatus side is associated with a second connection established on a second apparatus side;

terminating a close instruction message, which is issued from a communication layer higher than the first and second connections and directed to a second apparatus, upon receiving the close instruction message from a first apparatus, and updating the control information in response to the receiving of the close instruction message; and

enabling, when a switchover notice indicating a switch from the first apparatus to a third apparatus is given and the control information indicates the receiving of the close instruction message, a third connection established on a third apparatus side and updating the control information in such a manner that the second connection is associated with the third connection.