For each service of the protocol, the backend interface declares one or two functions. You are required to provide implementations of the functions representing the services that you wish to implement.
bend_initresult (*bend_init)(bend_initrequest *r);
    
     This handler is called once for each new connection request, after
     a new process/thread has been created, and an Initialize Request has
     been received from the client. The pointer to the
     bend_init handler is passed in the call to
     statserv_start.
    
This handler is also called when operating in SRU mode - when a connection has been made (even though SRU does not offer this service).
     Unlike previous versions of YAZ, the bend_init also
     serves as a handler that defines the Z39.50 services that the backend
     wish to support. Pointers to all service handlers,
     including search - and fetch must be specified here in this handler.
    
The request - and result structures are defined as
typedef struct bend_initrequest
{
    /** \brief user/name/password to be read */
    Z_IdAuthentication *auth; 
    /** \brief encoding stream (for results) */
    ODR stream;
    /** \brief printing stream */
    ODR print;
    /** \brief decoding stream (use stream for results) */
    ODR decode; 
    /** \brief reference ID */
    Z_ReferenceId *referenceId;
    /** \brief peer address of client */
    char *peer_name;           
    
    /** \brief character set and language negotiation 
    see include/yaz/z-charneg.h 
    */
    Z_CharSetandLanguageNegotiation *charneg_request;
    /** \brief character negotiation response */
    Z_External *charneg_response;
    /** \brief character set (encoding) for query terms 
        
    This is NULL by default. It should be set to the native character
    set that the backend assumes for query terms */
    char *query_charset;      
    /** \brief whehter query_charset also applies to recors 
    
    Is 0 (No) by default. Set to 1 (yes) if records is in the same
    character set as queries. If in doubt, use 0 (No).
    */
    int records_in_same_charset;
    char *implementation_id;
    char *implementation_name;
    char *implementation_version;
    /** \brief Z39.50 sort handler */
    int (*bend_sort)(void *handle, bend_sort_rr *rr);
    /** \brief SRU/Z39.50 search handler */
    int (*bend_search)(void *handle, bend_search_rr *rr);
    /** \brief SRU/Z39.50 fetch handler */
    int (*bend_fetch)(void *handle, bend_fetch_rr *rr);
    /** \brief SRU/Z39.50 present handler */
    int (*bend_present)(void *handle, bend_present_rr *rr);
    /** \brief Z39.50 extended services handler */
    int (*bend_esrequest) (void *handle, bend_esrequest_rr *rr);
    /** \brief Z39.50 delete result set handler */
    int (*bend_delete)(void *handle, bend_delete_rr *rr);
    /** \brief Z39.50 scan handler */
    int (*bend_scan)(void *handle, bend_scan_rr *rr);
    /** \brief Z39.50 segment facility handler */
    int (*bend_segment)(void *handle, bend_segment_rr *rr);
    /** \brief SRU explain handler */
    int (*bend_explain)(void *handle, bend_explain_rr *rr);
    /** \brief SRU scan handler */
    int (*bend_srw_scan)(void *handle, bend_scan_rr *rr);
    /** \brief SRU record update handler */
    int (*bend_srw_update)(void *handle, bend_update_rr *rr);
    /** \brief whether named result sets are supported (0=disable, 1=enable) */
    int named_result_sets;
} bend_initrequest;
typedef struct bend_initresult
{
    int errcode;               /* 0==OK */
    char *errstring;           /* system error string or NULL */
    void *handle;              /* private handle to the backend module */
} bend_initresult;
    
     In general, the server frontend expects that the
     bend_*result pointer that you return is valid at
     least until the next call to a bend_* function.
     This applies to all of the functions described herein. The parameter
     structure passed to you in the call belongs to the server frontend, and
     you should not make assumptions about its contents after the current
     function call has completed. In other words, if you want to retain any
     of the contents of a request structure, you should copy them.
    
     The errcode should be zero if the initialization of
     the backend went well. Any other value will be interpreted as an error.
     The errstring isn't used in the current version, but
     one option would be to stick it in the initResponse as a VisibleString.
     The handle is the most important parameter. It should
     be set to some value that uniquely identifies the current session to
     the backend implementation. It is used by the frontend server in any
     future calls to a backend function.
     The typical use is to set it to point to a dynamically allocated state
     structure that is private to your backend module.
    
     The auth member holds the authentication information
     part of the Z39.50 Initialize Request. Interpret this if your serves
     requires authentication. 
    
     The members peer_name,
     implementation_id,
     implementation_name and
     implementation_version holds
     DNS of client, ID of implementor, name
     of client (Z39.50) implementation - and version.
    
     The bend_ - members are set to NULL when
     bend_init is called. Modify the pointers by
     setting them to point to backend functions.
    
We now describe the handlers that are required to support search - and retrieve. You must support two functions - one for search - and one for fetch (retrieval of one record). If desirable you can provide a third handler which is called when a present request is received which allows you to optimize retrieval of multiple-records.
int (*bend_search) (void *handle, bend_search_rr *rr);
typedef struct {
    char *setname;             /* name to give to this set */
    int replace_set;           /* replace set, if it already exists */
    int num_bases;             /* number of databases in list */
    char **basenames;          /* databases to search */
    Z_ReferenceId *referenceId;/* reference ID */
    Z_Query *query;            /* query structure */
    ODR stream;                /* encode stream */
    ODR decode;                /* decode stream */
    ODR print;                 /* print stream */
    bend_request request;
    bend_association association;
    int *fd;
    int hits;                  /* number of hits */
    int errcode;               /* 0==OK */
    char *errstring;           /* system error string or NULL */
    Z_OtherInformation *search_info; /* additional search info */
    char *srw_sortKeys;        /* holds SRU/SRW sortKeys info */
    char *srw_setname;         /* holds SRU/SRW generated resultsetID */
    int *srw_setnameIdleTime;  /* holds SRU/SRW life-time */
    int estimated_hit_count;   /* if hit count is estimated */
    int partial_resultset;     /* if result set is partial */
} bend_search_rr;
    
     The bend_search handler is a fairly close
     approximation of a protocol Z39.50 Search Request - and Response PDUs
     The setname is the resultSetName from the protocol.
     You are required to establish a mapping between the set name and whatever
     your backend database likes to use.
     Similarly, the replace_set is a boolean value
     corresponding to the resultSetIndicator field in the protocol.
     num_bases/basenames is a length of/array of character
     pointers to the database names provided by the client.
     The query is the full query structure as defined in
     the protocol ASN.1 specification.
     It can be either of the possible query types, and it's up to you to
     determine if you can handle the provided query type.
     Rather than reproduce the C interface here, we'll refer you to the
     structure definitions in the file
     include/yaz/z-core.h. If you want to look at the
     attributeSetId OID of the RPN query, you can either match it against
     your own internal tables, or you can use the 
     OID tools.
    
     The structure contains a number of hits, and an
     errcode/errstring pair. If an error occurs
     during the search, or if you're unhappy with the request, you should
     set the errcode to a value from the BIB-1 diagnostic set. The value
     will then be returned to the user in a nonsurrogate diagnostic record
     in the response. The errstring, if provided, will
     go in the addinfo field. Look at the protocol definition for the
     defined error codes, and the suggested uses of the addinfo field.
    
     The bend_search handler is also called when
     the frontend server receives a SRU SearchRetrieveRequest.
     For SRU, a CQL query is usually provided by the client.
     The CQL query is available as part of Z_Query
     structure (note that CQL is now part of Z39.50 via an external).
     To support CQL in existing implementations that only do Type-1,
     we refer to the CQL-to-PQF tool described
     here.
    
To maintain backwards compatibility, the frontend server of yaz always assume that error codes are BIB-1 diagnostics. For SRU operation, a Bib-1 diagnostic code is mapped to SRU diagnostic.
int (*bend_fetch) (void *handle, bend_fetch_rr *rr);
typedef struct bend_fetch_rr {
    char *setname;             /* set name */
    int number;                /* record number */
    Z_ReferenceId *referenceId;/* reference ID */
    Odr_oid *request_format;        /* format, transfer syntax (OID) */
    Z_RecordComposition *comp; /* Formatting instructions */
    ODR stream;                /* encoding stream - memory source if req */
    ODR print;                 /* printing stream */
    char *basename;            /* name of database that provided record */
    int len;                   /* length of record or -1 if structured */
    char *record;              /* record */
    int last_in_set;           /* is it?  */
    Odr_oid *output_format;        /* response format/syntax (OID) */
    int errcode;               /* 0==success */
    char *errstring;           /* system error string or NULL */
    int surrogate_flag;        /* surrogate diagnostic */
    char *schema;              /* string record schema input/output */
} bend_fetch_rr;
    
     The frontend server calls the bend_fetch handler
     when it needs database records to fulfill a Z39.50 Search Request, a
     Z39.50 Present Request or a SRU SearchRetrieveRequest.
     The setname is simply the name of the result set
     that holds the reference to the desired record.
     The number is the offset into the set (with 1
     being the first record in the set). The format field
     is the record format requested by the client (See
     Section 2, “Object Identifiers”).
     A value of NULL for format indicates that the
     client did not request a specific format.
     The stream argument is an ODR stream which
     should be used for allocating space for structured data records.
     The stream will be reset when all records have been assembled, and
     the response package has been transmitted.
     For unstructured data, the backend is responsible for maintaining a
     static or dynamic buffer for the record between calls.
    
     If a SRU SearchRetrieveRequest is received by the frontend server,
     the referenceId is NULL and the
     format (transfer syntax) is the OID for XML.
     The schema for SRU is stored in both the
     Z_RecordComposition
     structure and schema (simple string).
    
     In the structure, the basename is the name of the
     database that holds the
     record. len is the length of the record returned, in
     bytes, and record is a pointer to the record.
     last_in_set should be nonzero only if the record
     returned is the last one in the given result set.
     errcode and errstring, if
     given, will be interpreted as a global error pertaining to the
     set, and will be returned in a non-surrogate-diagnostic.
     If you wish to return the error as a surrogate-diagnostic
     (local error) you can do this by setting
     surrogate_flag to 1 also.
    
     If the len field has the value -1, then
     record is assumed to point to a constructed data
     type. The format field will be used to determine
     which encoder should be used to serialize the data.
    
      If your backend generates structured records, it should use
      odr_malloc() on the provided stream for allocating
      data: This allows the frontend server to keep track of the record sizes.
     
     The format field is mapped to an object identifier
     in the direct reference of the resulting EXTERNAL representation
     of the record.
    
The current version of YAZ only supports the direct reference mode.
int (*bend_present) (void *handle, bend_present_rr *rr);
typedef struct {
    char *setname;             /* set name */
    int start;
    int number;                /* record number */
    Odr_oid *format;           /* format, transfer syntax (OID) */
    Z_ReferenceId *referenceId;/* reference ID */
    Z_RecordComposition *comp; /* Formatting instructions */
    ODR stream;                /* encoding stream - memory source if required */
    ODR print;                 /* printing stream */
    bend_request request;
    bend_association association;
    int hits;                  /* number of hits */
    int errcode;               /* 0==OK */
    char *errstring;           /* system error string or NULL */
} bend_present_rr;
    
     The bend_present handler is called when
     the server receives a Z39.50 Present Request.
     The setname,
     start and number is the
     name of the result set - start position - and number of records to
     be retrieved respectively. format and
     comp is the preferred transfer syntax and element
     specifications of the present request.
    
     Note that this is handler serves as a supplement for
     bend_fetch and need not to be defined in order to
     support search - and retrieve. 
    
For back-ends that supports delete of a result set only one handler must be defined.
int (*bend_delete)(void *handle, bend_delete_rr *rr);
typedef struct bend_delete_rr {
    int function;
    int num_setnames;
    char **setnames;
    Z_ReferenceId *referenceId;
    int delete_status;      /* status for the whole operation */
    int *statuses;          /* status each set - indexed as setnames */
    ODR stream;
    ODR print; 
} bend_delete_rr;
    The delete set function definition is rather primitive, mostly because we have had no practical need for it as of yet. If someone wants to provide a full delete service, we'd be happy to add the extra parameters that are required. Are there clients out there that will actually delete sets they no longer need?
For servers that wish to offer the scan service one handler must be defined.
int (*bend_scan)(void *handle, bend_scan_rr *rr);
typedef enum {
    BEND_SCAN_SUCCESS,  /* ok */
    BEND_SCAN_PARTIAL   /* not all entries could be found */
} bend_scan_status;
typedef struct bend_scan_rr {
    int num_bases;      /* number of elements in databaselist */
    char **basenames;   /* databases to search */
    Odr_oid *attributeset;
    Z_ReferenceId *referenceId; /* reference ID */
    Z_AttributesPlusTerm *term;
    ODR stream;         /* encoding stream - memory source if required */
    ODR print;          /* printing stream */
    int *step_size;     /* step size */
    int term_position;  /* desired index of term in result list/returned */
    int num_entries;    /* number of entries requested/returned */
    /* scan term entries. The called handler does not have
       to allocate this. Size of entries is num_entries (see above) */
    struct scan_entry *entries;
    bend_scan_status status;
    int errcode;
    char *errstring;
    char *scanClause;   /* CQL scan clause */
    char *setname;      /* Scan in result set (NULL if omitted) */
} bend_scan_rr;
    
    This backend server handles both Z39.50 scan 
    and SRU scan. In order for a handler to distinguish between SRU (CQL) scan 
    Z39.50 Scan , it must check for a non-NULL value of 
    scanClause.
   
if designed today, it would be a choice using a union or similar, but that would break binary compatibility with existing servers.