- <chapter id="proxy">
- <title>The YAZ Proxy</title>
- <para>
- The YAZ proxy is a transparent SRW/SRU/Z39.50-to-Z39.50 gateway.
- That is, it is a SRW/SRU/Z39.50 server which has as its back-end a
- Z39.50 client that forwards requests on to another server (known as
- the <firstterm>backend target</firstterm>.)
- </para>
- <para>
- -- All config directives --
- -- SRW/SRU ..
- -- Example config
- -- Mention XSLT conversion
- </para>
- <para>
- The YAZ Proxy is useful for debugging SRW/SRU/Z39.50 software, logging
- APDUs, redirecting Z39.50 packages through firewalls, etc.
- Furthermore, it offers facilities that often
- boost performance for connectionless Z39.50 clients such
- as web gateways.
- </para>
- <para>
- Unlike most other server software, the proxy runs single-threaded,
- single-process. Every I/O operation
- is non-blocking so it is very lightweight and extremely fast.
- It does not store any state information on the hard drive,
- except any log files you ask for.
- </para>
-
- <section id="proxy-example">
- <title>Example: Using the Proxy to Log APDUs</title>
- <para>
- Suppose you use a commercial Z39.50 client for which you do not
- have source code, and it's not behaving how you think it should
- when running against some specific server that you have no control
- over. One way to diagnose the problem is to find out what packets
- (APDUs) are being sent and received, but not all client
- applications have facilities to do APDU logging.
- </para>
- <para>
- No problem. Run the proxy on a friendly machine, get it to log
- APDUs, and point the errant client at the proxy instead of
- directly at the server that's causing it problems.
- </para>
- <para>
- Suppose the server is running on <literal>foo.bar.com</literal>,
- port 18398. Run the proxy on the machine of your choice, say
- <literal>your.company.com</literal> like this:
- </para>
- <screen>
- yazproxy -a - -t tcp:foo.bar.com:18398 tcp:@:9000
- </screen>
- <para>
- (The <literal>-a -</literal> option requests APDU logging on
- standard output, <literal>-t tcp:foo.bar.com:18398</literal>
- specifies where the backend target is, and
- <literal>tcp:@:9000</literal> tells the proxy to listen on port
- 9000 and accept connections from any machine.)
- </para>
- <para>
- Now change your client application's configuration so that instead
- of connecting to <literal>foo.bar.com</literal> port 18398, it
- connects to <literal>your.company.com</literal> port 9000, and
- start it up. It will work exactly as usual, but all the packets
- will be sent via the proxy, which will generate a log like this:
- </para>
- <screen><![CDATA[
- decode choice
- initRequest {
- referenceId OCTETSTRING(len=4) 69 6E 69 74
- protocolVersion BITSTRING(len=1)
- options BITSTRING(len=2)
- preferredMessageSize 1048576
- maximumRecordSize 1048576
- implementationId 'Mike Taylor (id=169)'
- implementationName 'Net::Z3950.pm (Perl)'
- implementationVersion '0.31'
- }
- encode choice
- initResponse {
- referenceId OCTETSTRING(len=4) 69 6E 69 74
- protocolVersion BITSTRING(len=1)
- options BITSTRING(len=2)
- preferredMessageSize 1048576
- maximumRecordSize 1048576
- result TRUE
- implementationId '81'
- implementationName 'GFS/YAZ / Zebra Information Server'
- implementationVersion 'YAZ 1.9.1 / Zebra 1.3.3'
- }
- decode choice
- searchRequest {
- referenceId OCTETSTRING(len=1) 30
- smallSetUpperBound 0
- largeSetLowerBound 1
- mediumSetPresentNumber 0
- replaceIndicator TRUE
- resultSetName 'default'
- databaseNames {
- 'gils'
- }
- {
- smallSetElementSetNames choice
- generic 'F'
- }
- {
- mediumSetElementSetNames choice
- generic 'B'
- }
- preferredRecordSyntax OID: 1 2 840 10003 5 10
- {
- query choice
- type_1 {
- attributeSetId OID: 1 2 840 10003 3 1
- RPNStructure choice
- {
- simple choice
- attributesPlusTerm {
- attributes {
- }
- term choice
- general OCTETSTRING(len=7) 6D 69 6E 65 72 61 6C
- }
- }
- }
- }
- }
-]]>
- </screen>
- </section>
-
- <section id="proxy-target">
- <title>Specifying the Backend Target</title>
- <para>
- When the proxy receives a Z39.50 Initialize Request from a Z39.50
- client, it determines the backend target by the following rules:
- <orderedlist>
- <listitem>
- <para>If the <literal>InitializeRequest</literal> PDU from the
- client includes an
- <link linkend="otherinfo-encoding"><literal>otherInfo</literal></link>
- element with OID
- <literal>1.2.840.10003.10.1000.81.1</literal>, then the
- contents of that element specify the target to be used, in the
- usual YAZ address format (typically
- <literal>tcp:<parameter>hostname</parameter>:<parameter>port</parameter></literal>)
- as described in
- <ulink url="http://www.indexdata.dk/yaz/doc/comstack.addresses.tkl"
- >the Addresses section of the YAZ manual</ulink>.
- </para>
- </listitem>
- <listitem>
- <para>Otherwise, the Proxy uses the default target, if one was
- specified on the command-line with the <literal>-t</literal>
- option. A default target can also be specified in the
- XML Config file.
- </para>
- </listitem>
- <listitem>
- <para>Otherwise, the proxy closes the connection with
- the client.
- </para>
- </listitem>
- </orderedlist>
- </para>
- </section>
- <section id="proxy-keepalive">
- <title>Keep-alive Facility</title>
- <para>
- The keep-alive is a facility where the proxy keeps the connection to the
- backend - even if the client closes the connection to the proxy.
- </para>
- <para>
- If a new or another client connects to the proxy again and requests the
- same backend it will be reassigned to this backend. In this case, the
- proxy sends an initialize response directly to the client and an
- initialize handshake with the backend is omitted.
- </para>
- <para>
- When a client reconnects, query and record caching works better, if the
- proxy assigns it to the same backend as before. And the result set
- (if any) is re-used. To achieve this, Index Data defined a session
- cookie which identifies the backend session.
- </para>
- <para>
- The cookie is defined by the client and is sent as part of the
- Initialize Request and passed in an
- <link linkend="otherinfo-encoding"><literal>otherInfo</literal></link>
- element with OID <literal>1.2.840.10003.10.1000.81.2</literal>.
- </para>
- <para>
- Clients that do not send a cookie as part of the initialize request
- may still better performance, since the init handshake is saved.
- </para>
- </section>
-
- <section id="query-cache">
- <title>Query Caching</title>
- <para>
- Simple stateless clients often send identical Z39.50 searches
- in a relatively short period of time (e.g. in order to produce a
- results-list page, the next page,
- a single full-record, etc). And for many targets, it's
- much more expensive to produce a new result set than to
- reuse an existing one.
- </para>
- <para>
- The proxy tries to solve that by remembering the last query for each
- backend target, so that if an identical query is received next, it
- is turned into Present Requests rather than new Search Requests.
- </para>
- <note>
- <para>
- In a future we release will will probably allows for
- an arbitrary-sized cache for targets supporting named result sets.
- </para>
- </note>
- <para>
- You can enable/disable query caching using option -o.
- </para>
- </section>
-
- <section id="record-cache">
- <title>Record Caching</title>
- <para>
- As an option, the proxy may also cache result set records for the
- last search.
- The proxy takes into account the Record Syntax and CompSpec.
- The CompSpec includes simple element set names as well.
- By default the cache is 200000 bytes per session.
- </para>
- </section>
-
- <section id="query-validation">
- <title>Query Validation</title>
- <para>
- The Proxy may also be configured to trap particular attributes in
- Type-1 queries and send Bib-1 diagnostics back to the client without
- even consulting the backend target. This facility may be useful if
- a target does not properly issue diagnostics when unsupported attributes
- are send to it.
- </para>
- </section>
-
- <section id="record-validation">
- <title>Record Syntax Validation</title>
- <para>
- The proxy may be configured to accept, reject or convert records.
- When accepted, the target passes search/present requests to the
- backend target under the assumption that the target can honor the
- request (In fact it may not do that). When a record is rejected because
- the record syntax is "unsupported" the proxy returns a diagnostic to the
- client. Finally, the proxy may convert records.
- </para>
- <para>
- The proxy can convert from MARC to MARCXML and thereby offer an
- XML version of any MARC record as long as it is ISO2709 encoded.
- If the proxy is compiled with libXSLT support it can also
- perform XSLT on XML.
- </para>
- </section>
-
- <section id="other-optimizations">
- <title>Other Optimizations</title>
- <para>
- We've had some plans to support global caching of result set records,
- but this has not yet been implemented.
- </para>
- </section>
-
- <section id="proxy-config-file">
- <title>Proxy Configuration File</title>
- <para>
- The Proxy may read a configuration file using option
- <literal>-c</literal> followed by the filename of a config file.
- </para>
- <para>
- The config file is XML based. The YAZ proxy must be compiled
- with <ulink url="http://www.xmlsoft.org/">libxml2</ulink> and
- <ulink url="http://xmlsoft.org/XSLT/">libXSLT</ulink> support in
- order for the config file facility to be enabled.
- </para>
- <tip>
- <para>To check for a config file to be well-formed, the yazproxy may
- be invoked without specifying a listening port, i.e.
- <screen>
- yazproxy -c myconfig.xml
- </screen>
- If this does not produce errors, the file is well-formed.
- </para>
- </tip>
- <section id="proxy-config-header">
- <title>Proxy Configuration Header</title>
- <para>
- The proxy config file must have a root element called
- <literal>proxy</literal>. All information except an optional XML
- header must be stored within the <literal>proxy</literal> element.
- </para>
- <screen>
- <?xml version="1.0"?>
- <proxy>
- <!-- content here .. -->
- </proxy>
- </screen>
- </section>
- <section id="proxy-config-target">
- <title>Configuration: target</title>
- <para>
- The element <literal>target</literal> which may be repeated zero
- or more times with parent element <literal>proxy</literal> contains
- information about each backend target.
- The <literal>target</literal> element have two attributes:
- <literal>name</literal> which holds the logical name of the backend
- target (required) and <literal>default</literal> (optional) which
- (when given) specifies that the backend target is the default target -
- equivalent to command line option <literal>-t</literal>.
- </para>
- <para>
- <screen>
- <?xml version="1.0"?>
- <proxy>
- <target name="server1" default="1">
- <!-- description of server1 .. -->
- </target>
- <target name="server2">
- <!-- description of server2 .. -->
- </target>
- </proxy>
- </screen>
- </para>
- </section>
- <section id="proxy-config-url">
- <title>Configuration:url</title>
- <para>
- The <literal>url</literal> which may be repeated one or more times
- should be the child of the <literal>target</literal> element.
- The CDATA of <literal>url</literal> is the Z-URL of the backend.
- </para>
- <para>
- Multiple <literal>url</literal> element may be used. In that case, then
- a client initiates a session, the proxy chooses the URL with the lowest
- number of active sessions, thereby distributing the load. It is
- assumed that each URL represents the same database (data).
- </para>
- </section>
- <section id="proxy-config-keepalive">
- <title>Configuration: keepalive</title>
- <para>The <literal>keepalive</literal> element holds information about
- the keepalive Z39.50 sessions. Keepalive sessions are proxy-to-backend
- sessions that is no longer associated with a client session.
- </para>
- <para>The <literal>keepalive</literal> element which is the child of
- the <literal>target</literal>holds two elements:
- <literal>bandwidth</literal> and <literal>pdu</literal>.
- The <literal>bandwidth</literal> is the maximum total bytes
- transferred to/from the target. If a target session exceeds this
- limit, it is shut down (and no longer kept alive).
- The <literal>pdu</literal> is the maximum number of requests sent
- to the target. If a target session exceeds this limit, it is
- shut down. The idea of these two limits is that avoid very long
- sessions that use resources in a backend (that leaks!).
- </para>
- <para>
- The following sets maximum number of bytes transferred in a
- target session to 1 MB and maxinum of requests to 400.
- <screen>
- <keepalive>
- <bandwidth>1048576</bandwidth>
- <retrieve>400</retrieve>
- </keepalive>
- </screen>
- </para>
- </section>
- <section id="proxy-config-limit">
- <title>Configuration: limit</title>
- <para>
- The <literal>limit</literal> section specifies bandwidth/pdu requests
- limits for an active session.
- The proxy records bandwidth/pdu requests during the last 60 seconds
- (1 minute). The <literal>limit</literal> may include the
- elements <literal>bandwidth</literal>, <literal>pdu</literal>,
- and <literal>retrieve</literal>. The <literal>bandwidth</literal>
- measures the number of bytes transferred within the last minute.
- The <literal>pdu</literal> is the number of requests in the last
- minute. The <literal>retrieve</literal> holds the maximum records to
- be retrieved in one Present Request.
- </para>
- <para>
- If a bandwidth/pdu limit is reached the proxy will postpone the
- requests to the target and wait one or more seconds. The idea of the
- limit is to ensure that clients that downloads hundreds or thousands of
- records do not hurt other users.
- </para>
- <para>
- The following sets maximum number of bytes transferred per minute to
- 500Kbytes and maximum number of requests to 40.
- <screen>
- <limit>
- <bandwidth>524288</bandwidth>
- <retrieve>40</retrieve>
- </limit>
- </screen>
- </para>
- <note>
- <para>
- Typically the limits for keepalive are much higher than
- those for session minute average.
- </para>
- </note>
- </section>
-
- <section id="proxy-config-attribute">
- <title>Configuration: attribute</title>
- <para>
- The <literal>attribute</literal> element specifies accept or reject
- or a particular attribute type, value pair.
- Well-behaving targets will reject unsupported attributes on their
- own. This feature is useful for targets that do not gracefully
- handle unsupported attributes.
- </para>
- <para>
- Attribute elements may be repeated. The proxy inspects the attribute
- specifications in the order as specified in the configuration file.
- When a given attribute specification matches a given attribute list
- in a query, the proxy takes appropriate action (reject, accept).
- </para>
- <para>
- If no attribute specifications matches the attribute list in a query,
- it is accepted.
- </para>
- <para>
- The <literal>attribute</literal> element has two required attributes:
- <literal>type</literal> which is the Attribute Type-1 type, and
- <literal>value</literal> which is the Attribute Type-1 value.
- The special value/type <literal>*</literal> matches any attribute
- type/value. A value may also be specified as a list with each
- value separated by comma, a value may also be specified as a
- list: low value - dash - high value.
- </para>
- <para>
- If attribute <literal>error</literal> is given, that holds a
- Bib-1 diagnostic which is sent to the client if the particular
- type, value is part of a query.
- </para>
- <para>
- If attribute <literal>error</literal> is not given, the attribute
- type, value is accepted and passed to the backend target.
- </para>
- <para>
- A target that supports use attributes 1,4, 1000 through 1003 and
- no other use attributes, could use the following rules:
- <screen>
- <attribute type="1" value="1,4,1000-1003">
- <attribute type="1" value="*" error="114"/>
- </screen>
- </para>
- </section>
-
- <section id="proxy-config-syntax">
- <title>Configuration: syntax</title>
- <para>
- The <literal>syntax</literal> element specifies accept or reject
- or a particular record syntax request from the client.
- </para>
- <para>
- The <literal>syntax</literal> has one required attribute:
- <literal>type</literal> which is the Preferred Record Syntax.
- </para>
- <para>
- If attribute <literal>error</literal> is given, that holds a
- Bib-1 diagnostic which is sent to the client if the particular
- record syntax is part of a present - or search request.
- </para>
- <para>
- If attribute <literal>error</literal> is not given, the record syntax
- is accepted and passed to the backend target.
- </para>
- <para>
- If attribute <literal>marcxml</literal> is given, the proxy will
- perform MARC21 to MARCXML conversion. In this case the
- <literal>type</literal> should be XML. The proxy will use
- preferred record syntax USMARC/MARC21 against the backend target.
- </para>
- <para>To accept USMARC and offer MARCXML XML records but reject
- all other requests the following configuration could be used:
- <screen>
- <proxy>
- <target name="mytarget">
- <syntax type="usmarc"/>
- <syntax type="xml" marcxml="1"/>
- <syntax type="*" error="238"/>
- </target>
- </proxy>
- </screen>
- </para>
- </section>
-
- <section id="proxy-config-target-timeout">
- <title>Configuration: target-timeout</title>
- <para>
- The element <literal>target-timeout</literal> is the child of element
- <literal>target</literal> and specifies the amount in seconds before
- a target session is shut down.
- </para>
- <para>
- This can also be specified on the command line by using option
- <literal>-T</literal>. Refer to <xref linkend="proxy-usage"/>.
- </para>
- </section>
-
- <section id="proxy-config-client-timeout">
- <title>Configuration: client-timeout</title>
- <para>
- The element <literal>client-timeout</literal> is the child of element
- <literal>target</literal> and specifies the amount in seconds before
- a client session is shut down.
- </para>
- <para>
- This can also be specified on the command line by using option
- <literal>-i</literal>. Refer to <xref linkend="proxy-usage"/>.
- </para>
- </section>
-
- <section id="proxy-config-preinit">
- <title>Configuration: preinit</title>
- <para>
- The element <literal>preinit</literal> is the child of element
- <literal>target</literal> and specifies the number of spare
- connection to a target. By default no spare connection are
- created by the proxy. If the proxy uses a target exclusive or
- a lot, the preinit session will ensure that target sessions
- have been made before the client makes a connection and will therefore
- reduce the connect-init handshake dramatically. Never set this to
- more than 5.
- </para>
- </section>
-
- <section id="proxy-config-max-clients">
- <title>Configuration: max-clients</title>
- <para>
- The element <literal>max-clients</literal> is the child of element
- <literal>proxy</literal> and specifies the total number of
- allowed connections to targets (all targets). If this limit
- is reached the proxy will close the least recently used connection.
- </para>
- <para>
- Note, that many Unix systems impose a system on the number of
- open files allowed in a single process, typically in the
- range 256 (Solaris) to 1024 (Linux).
- The proxy uses 2 sockets per session + a few files
- for logging. As a rule of thumb, ensure that 2*max-clients + 5
- can be opened by the proxy process.
- </para>
- <tip>
- <para>
- Using the <ulink url="http://www.gnu.org/software/bash/bash.html">
- bash</ulink> shell, you can set the limit with
- <literal>ulimit -n</literal><replaceable>no</replaceable>.
- Use <literal>ulimit -a</literal> to display limits.
- </para>
- </tip>
- </section>
-
- <section id="proxy-config-log">
- <title>Configuration: log</title>
- <para>
- The element <literal>log</literal> is the child of element
- <literal>proxy</literal> and specifies what to be logged by the
- proxy.
- </para>
- <para>
- Specify the log file with command-line option <literal>-l</literal>.
- </para>
- <para>
- The text of the <literal>log</literal> element is a sequence of
- options separated by white space. See the table below:
- <table frame="top"><title>Logging options</title>
- <tgroup cols="2">
- <colspec colwidth="1*" colname="option"/>
- <colspec colwidth="2*" colname="description"/>
- <thead>
- <row>
- <entry>Option</entry>
- <entry>Description</entry>
- </row>
- </thead>
- <tbody>
- <row>
- <entry><literal>client-apdu</literal></entry>
- <entry>
- Log APDUs as reported by YAZ for the
- communication between the client and the proxy.
- This facility is equivalent to the APDU logging that
- happens when using option <literal>-a</literal>, however
- this tells the proxy to log in the same file as given
- by <literal>-l</literal>.
- </entry>
- </row>
- <row>
- <entry><literal>server-apdu</literal></entry>
- <entry>
- Log APDUs as reported by YAZ for the
- communication between the proxy and the server (backend).
- </entry>
- </row>
- <row>
- <entry><literal>clients-requests</literal></entry>
- <entry>
- Log a brief description about requests transferred between
- the client and the proxy. The name of the request and the size
- of the APDU is logged.
- </entry>
- </row>
- <row>
- <entry><literal>server-requests</literal></entry>
- <entry>
- Log a brief description about requests transferred between
- the proxy and the server (backend). The name of the request
- and the size of the APDU is logged.
- </entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- </para>
- <para>
- To log communication in details between the proxy and the backend, th
- following configuration could be used:
- <screen><![CDATA[
- <target name="mytarget">
- <log>server-apdu server-requests</log>
- </target>
-]]>
- </screen>
- </para>
- </section>
-
- </section>
- <section id="proxy-usage">
- <title>Proxy Usage</title>
- <para>
- </para>
- <refentry id="yazproxy-ref">
- &yaz-proxy-ref;
- </refentry>
- </section>
- <section id="otherinfo-encoding"><title>OtherInformation Encoding</title>
- <para>
- The proxy uses the OtherInformation definition to carry
- information about the target address and cookie.
- </para>
- <screen>
- OtherInformation ::= [201] IMPLICIT SEQUENCE OF SEQUENCE{
- category [1] IMPLICIT InfoCategory OPTIONAL,
- information CHOICE{
- characterInfo [2] IMPLICIT InternationalString,
- binaryInfo [3] IMPLICIT OCTET STRING,
- externallyDefinedInfo [4] IMPLICIT EXTERNAL,
- oid [5] IMPLICIT OBJECT IDENTIFIER}}
---
- InfoCategory ::= SEQUENCE{
- categoryTypeId [1] IMPLICIT OBJECT IDENTIFIER OPTIONAL,
- categoryValue [2] IMPLICIT INTEGER}
- </screen>
- <para>
- The <literal>categoryTypeId</literal> is either
- OID 1.2.840.10003.10.1000.81.1, 1.2.840.10003.10.1000.81.2
- for proxy target and proxy cookie respectively. The
- integer element <literal>category</literal> is set to 0.
- The value proxy and cookie is stored in element
- <literal>characterInfo</literal> of the <literal>information</literal>
- choice.
- </para>
- </section>
- </chapter>
- <!-- Keep this comment at the end of the file
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-
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