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Implementing Remote ProcedureCalls

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how does a caller determine the machine address of the callee and specify to the callee the procedure to be invoked? The first is primarily a question of naming and the second a question of location.

2.1 Naming

The binding operation offered by our RPC package is to bind an importer of an interface to an exporter of an interface. After binding, calls made by the importer invoke procedures implemented by the (remote) exporter. There are two parts to the name of an interface: the type and the instance. The type is intended to specify, at some level of abstraction, which interface the caller expects the callee to implement. The instance is intended to specify which particular implementor of an abstract interface is desired. For example, the type of an interface might correspond to the abstraction of "mail server," and the instance would correspond to some particular mail server selected from many. A reasonable default for the type of an interface might be a name derived from the name of the Mesa interface module. Fundamentally, the semantics of an interface name are not dictated by the RPC package--they are an agreement between the exporter and the importer, not fully enforceable by the RPC package. However, the means by which an exporter uses the interface name to locate an exporter are dictated by the RPC

package, and these we now describe.

2.2 Locating an Appropriate Exporter We use the Grapevine distributed database 1

for our RPC binding. The major

attraction of using Grapevine is that it is widely and reliably available. Grapevine is distributed across multiple servers strategically located in our internet topology, and is configured to maintain at least three copies of each database entry. Since the Grapevine servers themselves are highly reliable and the data is replicated, it is extremely rare for us to be unable to look up a database entry. There are alternatives to using such a database, but we find them unsatisfactory. For example, we could include in our application programs the network addresses of the machine with which they wish to communicate: this would bind to a particular machine much too early for most applications. Alternatively, we could use some form of broadcast protocol to locate the desired machine: this would sometimes be acceptable, but as a general mechanism would cause too much interference with innocent bystanders, and would not be convenient for binding to machines not on the same local network.

Grapevine's database consists of a set of entries, each keyed by a character string known as a Grapevine RName. There are two varieties of entries: individ- ua/s and groups. Grapevine keeps several items of information for each database entry, but the RPC package is concerned with only two: for each individual there is a connect-site, which is a network address, and for each group there is a member-list, which is a list of RNames. The RPC package maintains two entries in the Grapevine database for each interface name: one for each type and one for each instance; so the type and instance are both Grapevine RNames. The database entry for the instance is a Grapevine individual whose connect-site is a network address, specifically, the network address of the machine on which that instance was last exported. The database entry for the type is a Grapevine group whose members are the Grapevine RNames of the instances of that type which

ACM Transactions on Computer Systems, Vol. 2, No. 1, February 1984

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