Advantages and disadvantages of different forms of operation

Table 1 summarizes the advantages and disadvantages of synchronous operations and the two general forms of asynchronous operations, ECB-posting and RPL exit-routine scheduling.

Table 1. Relative advantages of synchronous and asynchronous requests
Type of request	Performance	Storage requirements for RPLs and data areas	Programming complexity
Synchronous (OPTCD=SYN)	Adequate for batch-type program or for programs serving few simultaneous sessions.	Small, because only one request can be outstanding at a time. Can reuse RPL and data areas.	Simplest to code.
Asynchronous (OPTCD=ASY) ECB posting (ECB= address or INTERNAL)	Requires fewer system instructions than scheduling an RPL exit routine. The application can issue one WAIT for a combination of VTAM® and non-VTAM requests. ECBs are not automatically scheduled when the requested operation is complete.	Might require more storage because many pending requests can be outstanding, tying up RPLs and data areas.¹	Most complex.
Asynchronous (OPTCD=ASY) RPL exit routine scheduling (EXIT= address)	Requires more system instructions than posting an ECB. The RPL is automatically scheduled when the requested operation is complete. Some advantages if used to give priority of handling to a session (for example, first input after logon).	About the same as ECB posting.¹	Less complex than ECB posting.
Note: While an asynchronous request is outstanding, the RPL and associated data areas (for example, NIB, RPLAREA, EXTERNAL ECB, etc.) cannot be freed, altered, or reused.