Appendix A – Remote Database Access

Remote Database Access (RDA) is a standard for interfacing to databases via some network. The standards for networks are usually bound up with a hierarchy called the Open Systems Interconnect (OSI) layers, which look like this:

LAYERS (from top to bottom) WHAT'S DEFINED

---------------             the programs the end-users see
- Application -
---------------

----------------            negotiating syntax of messages
- Presentation -
----------------

----------------            naming, session establishment
- Session      -
----------------

----------------            acknowledgments, sequence numbers, time-outs
- Transport    -
----------------

----------------            routing through network addresses
- Network      -
----------------

----------------            packet structure
- Datalink     -
----------------

----------------            bit stream, transmission rate
- Physical     -
----------------

Typically, a DBMS programmer worries about Application, the top layer. RDA is a specification for the next lower layers – Presentation and Session. In other words: END USERS AND APPLICATION PROGRAMMERS HAVE NO DIRECT INTEREST IN RDA BECAUSE IT CONCERNS LOW-LEVEL CODING. Typically, the DBMS vendor supplies some mediating software which covers or obscures the network communication job.

All right: for most people, it’s not vital to understand RDA in any detail. What we think is useful is a short summary and tutorial. From this chapter, you’ll get an idea of:

  • What RDA’s job is.
  • What sort of information goes back and forth on the network.
  • What “client/server” actually means.

Table of Contents

ISO/IEC 9759

The standard for RDA is ISO/IEC 9759 “Remote Database Access For SQL”. This is a completely different document from the standard SQL document (ISO/IEC 9075), so it’s not correct to say that RDA is part of the SQL standard. But it’s a related standard, and the respective committees work together closely.

Much of the impetus for RDA came from an industry consortium known as the SQL Access Group (SAG). In 1991 members of the SAG consortium produced the first prototypes for version 1 of RDA. The latest edition is Version 3, which is still unofficial at the time we’re writing this. Version 3 is quite different from the earlier versions. It operates very much like the SQL/CLI specification, using similar or even the same names and parameters for most of its functions. For example, the SQL/CLI function SQLExecDirect has an RDA equivalent: RDAExecDirect. The parameters are similar (a handle and a string with an SQL statement and a size word). The effect is similar (an SQL statement is parsed and executed). The big difference, and the whole point of RDA, is architecture: SQLExecDirect is a function which you call directly, RDAExecDirect is a message which you pass to a “remote” entity.

The nearby functions are the “client”, the messages they send out are called “requests”. The remote entity is the “server”, the messages that it sends back are called “responses”.

TCP/IP

In the early RDA versions, the main concern was to interface via OSI. The latest version is quite specific: the main transport layer should be Transaction Connect Protocol / Internet Protocol, or TCP/IP. Actually TCP/IP is a family of protocols, made famous by the fact that The Internet runs on TCP/IP.

The TCP/IP protocol family is primarily responsible for the level “below” RDA – the Transport Layer. And, TCP/IP interfaces in its turn with the lower levels. The OSI stack works like a bucket brigade: the top level sends buckets down to the next level which passes to the next level and so on; meanwhile, buckets are coming back the other way. Which buckets are empty or full, depends on where the fire is at the moment.

The RDA is what’s between you (the application) and TCP/IP (the transport).

The Client/Server Model

The client sends requests to the server. The server sends responses to the client.

client ----------requests------------->
     <---------responses-------------   server

There are only eleven RDA requests. Each request designates a distinct operation that the client wants the server to perform. Here’s the list:

Request                  What the client wants the server to perform
RDAConnect               Set up connection between client and server
RDADisconnect            Cancel the connection made with RDAConnect
RDAEndTran               COMMIT, ROLLBACK, or "prepare for COMMIT"
RDAClientAttribute       Note what client's resource attributes are
RDAStatementPrepare      Prepare an SQL statement
RDAStatementDeallocate   Unprepare, close Cursor, free statement resource
RDAStatementExecute      Execute a prepared SQL statement
RDAStatementExecDirect   Prepare and execute a SQL statement in one go
RDAStatementFetchRows    FETCH selected rows and send contents to client
RDAStatementCloseCursor  No more selected rows will be fetched, clean up
RDAStatementCancel       Cancel any of the "RDAStatement..." requests

There is only one RDA response. A response designates the results of a server’s performance – or attempted performance – of a client request. Here’s the list:

Response                 What client should do with the response
"RDAResponse"            Read diagnostics and/or fetched row values

The eleven requests, and the response are described below. You may assume that all the messages contain identifiers and handles which are necessary to synchronize operations. We have omitted such “protocol” information in our descriptions. We have included only the “parameter” information, which is necessary to illustrate the essential point of each particular operation.

RDAConnect

Message Content:

DestinationServerName character string       name of server
UserName              character string       name of server
Authentication        character string       name of server

What the server is supposed to do:

Answer the phone! The client is sending enough information for the server to set up a connection – notice that the parameters here are the same as the parameters used for the SQL/CLI SQLConnect function.

The server will be responsible for a particular “cluster of Catalogs” – which we may think of as a “database”. There’s no standard way to pass a database name with this call, so we can assume that the usual expectation is that different databases will have different servers. In other words: by specifying the server, you are indirectly specifying the database that should be opened.

RDADisconnect

Message Content:

<nothing>

What the server is supposed to do:

Hang up the phone! Cut off the connection that was made with RDAConnect.

The disconnection will probably free the server so it can do other tasks, or perhaps shut down. The connection could be important for the network too – you’re freeing up a line so that the network server can allow other users to get on. But TCP/IP is stateless, so the concept of “freeing up a line” is less meaningful than it would be with other network protocols (such as NETBIOS).

RDAEndTran

Message Content:

CompletionType        32-bit INTEGER   flag: COMMIT OR ROLLBACK

What the server is supposed to:

The RDAEndTran marks a transaction boundary, and the flag will usually indicate COMMIT or ROLLBACK, so the usual job is simple: the server should end a current transaction with COMMIT or with ROLLBACK.

There is a reason that RDAEndTran is a distinct message – that makes it easier for the network to notice it, without having to know how to parse SQL statements. The network might intercept RDAEndTran, because the job of “committing” might affect more than just one server job.

One other flag value that CompletionType may contain (besides COMMIT or ROLLBACK) is: PREPARE TO COMMIT. As part of what we call the “two-phase commit” process, the first step is to co-ordinate: get all the servers on the network to acknowledge that they are all ready to commit. The second step is the COMMIT itself. Two-phase commit is advanced stuff; many DBMSs will refuse to support the PREPARE TO COMMIT operation.

RDAClientAttribute

Message Content:

ClientAttributes         array           List of attributes

What the server is supposed to do:

Take note of what attributes the client has. An “attribute” is an env or dbc or stmt attribute. That is, it’s a number or string that was earlier passed to the client, via a SQLSetEnvAttr or SQLSetConnectAttr or SQLSetStmtAttr function call.

The client won’t send a new RDAClientAttribute message every time its own attributes change. That would result in too much network traffic. One of the features of client/server architecture is: the server doesn’t do everything. The client is capable of storing data locally for a particular connection and only passing on what’s necessary when it’s necessary.

RDAStatementPrepare

Message Content:

StatementIdent                32-bit integer       sort of a handle
StatementText                 character string     SQL preparable statement

What the server is supposed to do:

Prepare an SQL statement. The client might be able to do some primitive syntax checking, but for the full job of binding and parsing, the client has to pass the SQL statement to the server. Only the server has direct access to the database and its metadata.

If we were calling SQLPrepare, we’d pass a hstmt (handle of a stmt). But a handle is a local identifier. There’s no guarantee that a handle value will be unique over the whole network. Therefore, instead of a hstmt, the client must pass a StatementIdent. This is a handle of a handle. Given a StatementIdent, the client and server can each look up what their respective hstmt values are for the same stmt.

RDAStatementDeallocate

Message Content:

StatementIdent          32-bit integer     sort of a handle

What the server is supposed to do:

Free a stmt. Compare the SQL/CLI function SQLFreeHandle(SQL_HANDLE_STMT,...).

With client-server, any statement resource (stmt) will be duplicated in two places: the client and the server. When the client is done with a stmt, it should send an RDAStatementDeallocate message to the server, so that the duplicate will be destroyed too.

RDAStatementExecute

Message Content:

StatementIdent                32-bit integer       sort of a handle
ParameterDescriptor           <array>              info re "host variable"
ParameterData                 <array>              "host variable" content

What the server is supposed to do:

Execute an SQL statement. Presumably the statement was prepared earlier, due to an RDAStatementPrepare message with the same StatementIdent.

The ParameterDescriptor and ParameterData fields – which weren’t necessary for SQLPrepare – represent the solution to a rather tough question: what should be done with input parameters? There are no direct RDA equivalents for the SQL/CLI functions that handle “parameter descriptors”. They’d be useless anyway, because SQL/CLI parameter descriptors require pointers and pointers have no meaning to a job on a different machine. So what happens is: the client bundles up all the input parameter values, and sends them as part of the message.

This might mean that requests get monstrous. But RDA’s designers thought it would be a bad idea to split RDAStatementExecute into multiple separate messages. What if the messages didn’t all arrive in the right order?

RDAStatementExecDirect

Message Content:

StatementIdent                32-bit integer       sort of a handle
StatementText                 character string     SQL preparable statement
ParameterDescriptor           <array>              info re "host variable"
ParameterData                 <array>              "host variable" content

What the Server is supposed to do:

Prepare what’s in StatementText, then execute it (using the input parameters if there are any).

RDAStatementExecDirect is logically redundant: we could accomplish the same thing by sending RDAStatementPrepare followed by RDAStatementExecute. Such redundancy is occasionally justifiable because it’s more efficient to send one message instead of two. Network traffic is expensive.

RDAStatementFetchRows

Message Content:

StatementIdent               32-bit integer   sort of a handle
FetchOrientation             32-bit integer   "next", "first", etc.
FetchOffset                  32-bit integer   start, relative to last fetch
FetchCount                   32-bit integer   how many rows to fetch

What the server is supposed to do:

Get the values in each of the indicated rows, and ship them to the client. Presumably there was an early RDAStatementExecute or RDAStatementExecDirect message for this StatementIdent, which caused a query to be executed. So the server has a result set that it can fetch the rows from.

The FetchOrientation and FetchOffset parameters contain values equivalent to those which are used by the SQL/CLI function SQLFetchScroll. For example, FetchOrientation might equal SQL_FETCH_ABSOLUTE and FetchOffset might equal 55.

In standard SQL, it’s only possible to fetch one row at a time. With RDA – and once again this can be explained by “efficiency” – it’s possible to ask for multiple rows to be fetched at once. That’s what the FetchCount parameter is for.

RDAStatementCloseCursor

Message Content:

StatementIdent               32-bit integer   sort of a handle

What the server is supposed to do:

Well, close the Cursor. This isn’t an end-of-transaction signal, but it does tell the server that it can forget about a result set that it created with a recent SELECT statement.

The client could pass messages in this sequence: (a) RDAStatementExecDirect with a SELECT statement so that the server creates a result set and Cursor, (b) RDAStatementFetchRows so that the server will fetch row values and send them to the client and (c) RDAStatementCloseCursor so that the server will free (and possibly unlock) resources that were allocated for Cursor maintenance.

RDAStatementCancel

Message Content:

StatementIdent               32-bit integer  sort of a handle

What the server is supposed to do:

Abort any activity that is taking place for the statement identified by StatementIdent. Since statement execution is atomic, the statement won’t be half-done – the result is as if the statement was never executed at all.

Why would the client send the statement in the first place, if it didn’t want to finish it? The usual answer is that the client can “change its mind”, perhaps because the execution is taking too long or because some new information has arrived (such as a Shutdown message from Windows). Or, consider this scenario:

  1. There are two servers. Each server is responsible for a copy of the same database.
  2. The client sends an RDAExecDirect message to both servers, with the same query.
  3. Inevitably, one of the servers will respond more quickly than the other. Perhaps it’s less busy, or perhaps its message packets are just moving by a shorter route.
  4. Once the client receives the response from the fast server, it sends an RDAStatementCancel message to the slow one, meaning “oh forget it – I already have the data”.
  5. The Server’s Response: RDAResponse

Message Content:

ServerAttributes            <array>   server's env/dbc/stmt attributes
Diagnostics                 <array>   errors or warnings
ParameterDescriptor         <array>   IPD, for RDAStatementPrepare
RowDescriptor               <array>   metadata, for RDAStatementFetchRows
Rows                        <array>   data, for RDAStatementFetchRows

The server sends one response message to the client for every request message that it receives from the client. The contents of responses can vary quite a bit, but usually you know what sort of responses to expect, by observing what the original request was. For example, if the request was an RDAStatementFetchRows message, a reasonable hope is that the response will contain data from the rows that got fetched.

A single response contains the entire set of results. This means that the contents of a response are useful for one or many operations that get performed by the client alone: get diagnostics, get attributes or get data. For example, the SQL/CLI function SQLGetDiagRec does not involve a network message containing RDAGetDiagRec – there is no such thing. Instead, when an application calls the SQLGetDiagRec function, the DBMS looks in the Diagnostics section of the last response that was received from the server. This means that SQLGetDiagRec can be called millions of times without affecting the network or the server. And the same goes for the other get functions. This also means that the client has to buffer all responses, but that’s okay – local storage is cheap.

A Short Session

Let’s walk through a short client/server session which involves a single query. We show client action on the left, server action on the right.

CLIENT                                SERVER

... Client is receiving input from    ... Server is idle, just waiting for
a user via a dialog box               incoming messages

Client sends RDAConnect message   -->

                                  <-- Server sends response, diagnostic="OK"

Client sends                      -->
RDAStatementExecDirect,
"SELECT * FROM Employees;" message

                                  <-- Server sends response, diagnostic="OK"

Client sends                      -->
RDAStatementFetchRows message

                                  <-- Server sends response, diagnostic="OK"
                                      and 2 rows: { ('A',1) ('B',2) }

Client sends                      -->
RDAStatementCloseCursor message

                                  <-- Server sends response, diagnostic="OK"

Client sends                      -->
RDAEndTran message

                                  <-- Server sends response, diagnostic="OK"

Client sends                      -->
RDADisconnect message

                                  <-- Server sends response, diagnostic="OK"

The network session is over, but      Server is idle again ...
the client can continue: "FETCH"
the first row, "FETCH" the second
row (from local buffers)...

Making it hard

In our initial tour of RDA, we skipped some non-essential points. We note them now so that you’ll at least be aware of the existence of anything that is potentially significant.

  1. Clients can send multiple requests without waiting for responses. This makes for hairy programming, because when responses eventually do arrive, they might not be in the right order.
  2. There might be multiple clients for one server, which leads to the various special considerations that we mentioned in our chapter on concurrency. Or, there might be several servers for one client.
  3. The network can be heterogeneous, that is, you should be able to connect a brand X client to a brand Y server. In theory.
  4. There is an optional client-side feature: a standard defined CLI (the “RDA API”). This is simply a wrapper, so that you can call a function which sends a message, without worrying about the details of “how to send a message” over TCP/IP.
  5. There are optional Schemas which contain Tables which contain information about available servers, logs of requests and so on. The Tables can be accessed in the same way that Views can be accessed in INFORMATION_SCHEMA.

What good is RDA?

In theory, RDA would be a relatively painless way to transfer information via TCP/IP or other OSI protocols. Programmers could write their own applications or utilities, such as Intranet servers. But most SQL programmers would prefer it if RDA was silently included in the DBMS. They want to call SQLExecDirect, for instance. If the DBMS does the job by silently sending RDAExecDirect messages to some server, fine.

So all a SQL programmer really needs to know is what you – we hope! – have just gleaned from this discussion of RDA. You should now be as competent as you need to be. Specifically:

  • You can appreciate database architecture now that you can visualize what “clients” and “servers” do.
  • You can write better SQL/CLI code since you know what some of the underlying effects are.
  • You can figure out whether direct use of RDA would help you around some specific network problem.

From those examples, it might seem that we’re talking about “low-level code” solutions – an analogue would be the way that people sometimes write assembler code snippets in their C programs. We’re big fans of that technique [[ footnote: see our book OPTIMIZING C WITH ASSEMBLY LANGUAGE, R&D Books ]]. But the analogy doesn’t really hold: writing for RDA is not inherently more arcane or low-level than writing any SQL/CLI code. That’s clear when you observe that the parameters are, in almost all cases, the same.