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## org.w3c.www.protocol.http.cache.push
Class PushCacheProtocol

    java.lang.Object
      |
      +--org.w3c.www.protocol.http.cache.push.PushCacheProtocol

public class PushCacheProtocol
extends java.lang.Object

PushCacheProtocol Characteristics of the protocol used to control the push cache, and methods for common operations

Protocol Description

To request that "/home/abc/page.html" is inserted in cache as "http://www.abc.com/page.html" the client sends a packet with command="ADD", and remain_len set to sizeof(add_packet_t) plus the sum of the lengths of the path and the urls including their null terminators. The client then sends an add_packet describing the lengths of the two strings followed by the path and then the url.

The server replies with either command="OK" and remain_len=0 or command="ERR" and remain_len set the the length of the error string that follows immediately. In the event of an "ERR" message the connection is closed by the server.

To request that the page associated with "http://www.abc.com/page.html" be removed from the cache the client sends a packet with command="DEL", and remain_len set to sizeof(int) plus the length of the url string including the trailing null character. The server replies as with ADD above. Attempting to remove a url that is not present in the cache results in an "OK" packet being returned, the cache is unchanged.

The client can ask if a url is present in the cache by sending a packet with command="PRS", and url information as with the DEL command. The server will reply with "OK" if the url is present, "NO" if the url is not present and "ERR" if an error was encountered.

The client can request that the cache be emptied of all urls by sending a packet with command="CLN" (clean). The remain_len field is set to zero. The server will reply with either OK or ERR.

The client can terminate the dialogue by sending a command="BYE" packet and then closing the connection.

'C' code describing the packet structures are shown below

     typedef struct {
                                 // Bytes  Notes
                                 // -----  -----
           char  tag[4];         // 0-3    = {'P','C','P','P'}
           short major_version;  // 4-5    = 1
           short minor_version;  // 6-7    = 1
           char  command[4];     // 8-11   Null terminated command string
           int   remain_len;     // 12-15  number of remaining bytes to read
     } packet_t;

     typedef struct {
           int   path_len;       // 4      Length of pathname (including null)
           int   url_len;        // 8      Length of URL (including null)
     } add_packet_t;

     Note that the command is always 4 characters in length and that the
     null characters are considered part of the command, so in Java (but
     not C) we must include the \0 when comparing strings:
       "ADD\0", "BYE\0", "OK\0\0", "ERR\0", "CLN\0", "PRS\0", "DEL\0"

Field Summary
`static int` | `ADD` Numeric codes for commands,
`static int` | `BYE` Numeric codes for commands,
`static int` | `CLN` Numeric codes for commands,
`static int` | `COMMAND_LEN` Size of command string in bytes (including null terminator)
`static int` | `DEL` Numeric codes for commands,
`static int` | `ERR` Numeric codes for commands,
`static int` | `HEADER_LEN` Combined size of tag and version information
`static short` | `MAJ_PROTO_VERSION` Protocol Major version
`static int` | `MAX_PAYLOAD_LEN` Maximum size of payload (follows basic packet)
`static int` | `MAX_STRING_LEN` Maximum size of strings (urls, paths, error messages)
`static short` | `MIN_PROTO_VERSION` Protocol minor version
`static int` | `NO` Numeric codes for commands,
`static int` | `NO_SUCH_COMMAND` Numeric codes for commands,
`static int` | `NOP` Numeric codes for commands,
`static int` | `OK` Numeric codes for commands,
`static int` | `PACKET_LEN` Size of basic packet in bytes
`static int` | `PRS` Numeric codes for commands,
`static int` | `TAG_LEN` Size of packet tag

Constructor Summary
`protected` | `PushCacheProtocol()` Singleton, no public constructor, use `instance()`

Method Summary
`byte[]` | `errorPacket(java.lang.String message)` Create error packet for specified error message
`byte[]` | `header()`
`static PushCacheProtocol` | `instance()` Access to single instance of this class
`boolean` | `isValidProtocolTag(byte[] packet)` True iff first four bytes of packet are identical to the protocol tag
`byte[]` | `noPacket()` Byte array for NO packet
`byte[]` | `okPacket()` Byte array for OK packet
`int` | `parseCommand(java.lang.String command)` Utility function for command string parsing

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### PACKET_LEN

    public static final int PACKET_LEN

  Size of basic packet in bytes

### COMMAND_LEN

    public static final int COMMAND_LEN

  Size of command string in bytes (including null terminator)

### HEADER_LEN

    public static final int HEADER_LEN

  Combined size of tag and version information

### TAG_LEN

    public static final int TAG_LEN

  Size of packet tag

### MAX_STRING_LEN

    public static final int MAX_STRING_LEN

  Maximum size of strings (urls, paths, error messages)

### MAX_PAYLOAD_LEN

    public static final int MAX_PAYLOAD_LEN

  Maximum size of payload (follows basic packet)

### MAJ_PROTO_VERSION

    public static final short MAJ_PROTO_VERSION

  Protocol Major version

### MIN_PROTO_VERSION

    public static final short MIN_PROTO_VERSION

  Protocol minor version

### NO_SUCH_COMMAND

    public static final int NO_SUCH_COMMAND

  Numeric codes for commands,

### ERR

    public static final int ERR

  Numeric codes for commands,

### ADD

    public static final int ADD

  Numeric codes for commands,

### DEL

    public static final int DEL

  Numeric codes for commands,

### CLN

    public static final int CLN

  Numeric codes for commands,

### PRS

    public static final int PRS

  Numeric codes for commands,

### BYE

    public static final int BYE

  Numeric codes for commands,

### OK

    public static final int OK

  Numeric codes for commands,

### NO

    public static final int NO

  Numeric codes for commands,

### NOP

    public static final int NOP

  Numeric codes for commands,

Constructor Detail

### PushCacheProtocol

    protected PushCacheProtocol()

  Singleton, no public constructor, use `instance()`
  See Also:
    `instance()`

Method Detail

### instance

    public static PushCacheProtocol instance()

  Access to single instance of this class

### parseCommand

    public int parseCommand(java.lang.String command)

  Utility function for command string parsing

### okPacket

    public byte[] okPacket()

  Byte array for OK packet

### noPacket

    public byte[] noPacket()

  Byte array for NO packet

### header

    public byte[] header()

### errorPacket

    public byte[] errorPacket(java.lang.String message)

  Create error packet for specified error message

### isValidProtocolTag

    public boolean isValidProtocolTag(byte[] packet)

  True iff first four bytes of packet are identical to the protocol tag

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