How It Is Identified: Message Tags
1st generation message passing systems used an integer “tag” (a.k.a. “type” or “id”) to match messages when received
- Most systems allowed wildcard on receive
- wildcard means match any tag i.e. any message
- Unsafe due to unexpected message arrival
- Most could match sender id, some with wildcards
- Wildcards unsafe; strict checks inconvenient
- All systems let users pick the tags
- Unsafe for libraries due to interference