Client/Server Communication - Protocol Capabilities Bit Mask
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During the authentication handshake, the client and the server exchange information on what the other is able or willing to do. This enables them to adjust their expectations of their peer and not send the data in some unsupported format. The exchange of information is accomplished through fields containing the bit mask of protocol capabilities.
The bit mask can be either 4 or 2 bytes long, depending on the context. The newer (4.1 and later) clients and servers understand 4-byte masks as well as 2-byte ones. The older (4.0 and earlier) ones can handle only 2-byte masks.
The server, regardless of the version, always announces its capabilities with a 2-byte bit mask. Although both newer clients and servers understand the 4-byte mask, the first packet in the dialog must be understood by any client regardless of the version. For this reason, even the newer clients expect the greeting packet to contain a 2-byte mask.
Once the client knows that it is talking to a newer server, it can announce its capabilities with a 4-byte mask. However, if the newer client detects that it is talking to an older server, it will announce the capabilities with only a 2-byte mask. Naturally, the older clients can only send a 2-byte mask; they are not aware of 4-byte ones.
Apparently was used in the early development of 4.1 to indicate that the server is able to use the new password format.
CLIENT_FOUND_ROWS
0x0002
Normally, in reporting the results of anUPDATEquery, the server returns the number of records that were actually modified. If this flag is set, the server is being asked to report the number of records that were matched by theWHEREclause. Not all of those will necessarily be updated, as some may already contain the desired values.
Please check back next week for the second part of the series.
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