Introduction Empire is designed as a smart server with dumb clients. An Empire client need to know nothing about the game. Even telnet would do. In fact, empire-client is little more than a slightly specialized telnet. In such a design, presentation is in the server, and it is designed for human consumption. Ideally, presentation and logic are cleanly separated, for easy selection between different presentations. There's no such separation in the Empire server, and separating them now would be a herculean effort. Thus, smart clients have to work with output designed for humans. That's not especially hard, just an awful lot of tedious work, and the result gets easily broken by minor changes in output format, which humans hardly notice. Instead of making smart clients parse output of commands designed for humans, one can add commands designed for machines. Such commands can share the code implementing game rules with their counterparts for humans. To do that cleanly means separating logic and presentation. Implementing them with their own copy of the code is no good --- how would you ensure that the two copies implement precisely the same game rules? Except for commands that actually don't do anything. There's a useful class of such commands: commands to show game configuration and state without altering it. The only game rules involved are those that govern who gets to see what. Ensuring that those are obeyed is tractable. Empire has had one such command since the beginning: dump. Empire 4.0.6 added more: sdump, ldump, pdump and ndump. 4.0.7 added lost and support for incremental dumps. These commands have served smart clients well. However, they cover just the most important part of the game state (sectors, ships, planes, land units, nukes), and no game configuration. They are not quite complete even for what they attempt to cover. Finally, their output is harder to parse than necessary. The xdump command is designed to be the dump to end all dumps. Like many good ideas, xdump has secondary uses. Dumping game state as text can be one half of a game export/import facility. Useful to migrate games to different machines or even, with some text mangling perhaps, different server versions. We will see below that xdump isn't quite sufficient for that, but it's a start. Means to import game configuration let you customize your game without recompiling the server. As we will see, configuration files have different requirements, which xdump doesn't satisfy without some extensions. If game import code can edit everything, then a deity command capable of editing everything is possible. Proof-of-concept code exists (not in the tree yet). Analysis of the data to dump Game state consists of a fixed set of table files (sector, ship, ...), telegram files, and a few miscellaneous files. Game configuration consists of a fixed set of configuration tables and scalar parameters. A table is an ordered set of records (table rows). All records have the same fields (table columns), which are statically typed. Fields may be integers, floating-point numbers, x- or y-coordinates, symbols and symbol sets encoded as integers in a way specific to the server version, and character arrays. Configuration table fields may be pointers to zero-terminated strings, null pointers allowed. No other pointers occur. Unions do not occur. Requirements analysis Requirements: * Capable to dump all tables. * Can do incremental dumps. * Output is text. * Output is reasonably compact. * Output is trivial to parse. Triviality test: if it's easy in AWK, C (no lex & yacc, just stdio), Lisp (just reader) and Perl (base language, no modules), then it's trivial enough. * Output identifies itself. * Output is self-contained; symbol encoding is explicit. * KISS: Keep it simple, stupid. Non-requirements: * Generality. We're not trying to design a general mechanism for dumping C data. * Completeness. We're not trying to dump stuff other than tables. * Abstraction. We're not trying to hide how things are stored in the server. When storage changes, xdump output will change as well, and consumers need to be updated. This is not because abstraction wouldn't be nice to have, just because we don't feel up to the task of designing one. Principles of Design Traditional dumps have a dump function for every table. These functions are simple, but exceedingly dull and repetitive. The selector code works differently. Each table has a descriptor, which among other things defines a dictionary of selector descriptors. A selector descriptor describes a field (table column) visible to players. This is what we call meta-data (data about data). The selector code knows nothing about the individual tables, it just interprets meta-data. That's smart, as it keeps the dull, repetitive parts in more easily maintainable meta-data rather than code. xdump follows the selector design, and uses the existing selector meta-data. This requires extending the meta-data to configuration tables, which weren't previously covered. It also requires some generalization of selector descriptors, so that all fields can be covered. To sum up, meta-data consists of a table of tables, and for each table a table of selectors (table of columns, so to speak). It is specific to the server version and how it is compiled on the host. To interpret a table xdump, you need its meta-data, because without it you have no idea what the columns mean. As meta-data is just a bunch of tables, xdump can dump it. But now you need meta-meta-data to make sense of the meta-data. Fortunately, meta-meta-data is the same for all xdumps, and therefore the recursion terminates with a single meta-meta-table. xdump dumps symbols and symbol sets as integers. To decode them, you need to know what the symbol numbers and symbol set bits mean. For this purpose, field meta-data includes the table ID of a symbol table. A symbol table is a table of value-name pairs, with the value in the leftmost column. You decode a symbol value by looking it up in the symbol table. You decode a symbol set value by looking up its bits (powers of two) in the symbol table. Some integer fields are actually keys in other tables. For instance, ship field type is a key in the table of ship types ship-chr, and plane field ship is a key in the ship table. Key -1 is special: it's a null key. Meta-data encodes these table reference just like for symbols: the meta-data has the ID of the referenced table, and that table has the key in the leftmost column. Obviously, that leftmost column is a table key as well, referencing the table itself. A table with its key in the leftmost column can be dumped partially. Without such a key, you need to count records to find the record index, and that works only if you can see a prefix of the complete table. Syntax of xdump command See info xdump. The xdump output Language Because the output is to be parsed by machines, it needs to be precisely specified. We use EBNF (ISO 14977) for syntax, except we use '-' in meta-identifiers and omit the concatenation symbol ','. table = header { record } footer ; header = "XDUMP" space [ "meta" space ] identifier space timestamp newline ; identifier = id-char1 { id-char } ; id-char1 = ? ASCII letter ? ; id-char = ? ASCII characters 33..126 except '"#()<>=' ? ; timestamp = intnum ; footer = "/" number newline ; record = [ fields ] newline ; fields = field { space field } ; field = intnum | flonum | string ; intnum = ? integer in printf %d format ? ; flonum = ? floating-point in printf %g format ? ; string = "nil" | '"' { str-char } '"' ; str-char = "\\" octal-digit octal-digit octal-digit | ? ASCII characters 33..126 except '"' and '\\' ? ; octal-digit = ? '0'..'7' ? ; space = ' ' ; newline = ? ASCII character 10 ? ; Notes: * The syntax for flonum is debatable. Precise conversion between floating-point and decimal is hard, and C libraries are not required to be precise. Using C99's %a format for flonum would avoid the issue, but some programming environments may have trouble converting that back to floating-point. We may change to %a anyway in the future. Clients are advised to accept both. * Strings syntax could perhaps profit from the remaining C escape sequences. Except for '\"': adding that would complicate regular expressions matching the string, and thus violate the `trivial to parse' requirement * Space is to be taken literally: a single space character. Not a non-empty sequence of white-space. Semantics: * The table identifier in the header is one of the names in xdump table. * The timestamp increases monotonically. It has a noticeable granularity: game state may change between an xdump and the next timestamp increase. If the table has a timestamp field, clients can xdump incrementally by using a conditional ?timestamp>T, where T is one less than the timestamp received with the last xdump of that table. Timestamp values are currently seconds since the epoch, but this might change, and clients are advised not to rely on it. * The number in the footer matches the number of records. * Fields match their meta-data (see Meta-Data below). * "nil" represents a null string (which is not the same as an empty string). Otherwise, fields are to be interpreted just like C literals. Meta-Data Table meta-data is in xdump table. Fields: * uid: The table ID, key for xdump table. IDs depend on the server version; clients should not hard-code them. This is the leftmost field. * name: The table name. Clients may identify tables by name. Field meta-data for table T is in xdump meta T. The order of fields in the xdump T matches the order of records in xdump meta T. Fields of xdump meta T are: * name: The field name. Matches the selector name. Clients may identify fields by name. This is the leftmost field. * type: The field's data type, a symbol. Clients should use this only as key for the symbol table. Symbols are: - "d", field uses intnum syntax - "g", field uses flonum syntax - "s", field uses string syntax - "c", field uses string syntax (only until version 4.3.33) * flags: The field's flags, a symbol set. Flags are: - "deity", field visible only to deities - "bits", field is a symbol set, field type must encode symbol "d", field table must not be -1. - "hidden", field value is masked for contact when option HIDDEN is enabled. Masked values are replaced by -1. * len: If non-zero, then the record encodes an array with that many elements. The array is dumped as len fields. Only until version 4.3.33: if field type encodes symbol "c", it is a character array, and is dumped as a single string field. * table: Key for xdump table. Unless -1, it defines the table referenced by the field value. Field type must encode symbol "d" then. Symbol table fields: * value: The symbol's encoding as integer. If the symbol can be element of a symbol set, this is a power of two. * name: The symbol's name. Notes on xdump Implementation Overall impact on the server code is low. To keeps xdump simple, storage of game state and game configuration tables has been unified under the common empfile abstraction, making nxtitem-iterators and selectors equally applicable to all tables. xdump required a few extensions to meta-data, which may become useful in other places as well: * Selectors can now deal with arrays (revived struct castr member ca_len). Not yet available on the Empire command line. * Selector meta-data can now express that a selector value is a key for another table (new struct castr member ca_table). The selector code doesn't use that, yet. * Redundant selectors can be marked so that xdump ignores them (new struct castr member ca_dump). Meta-data is in empfile[] (table meta-data), src/lib/global/nsc.c (selector meta-data), src/lib/global/symbol.c (symbol tables). The command is in src/lib/commands/xdump.c, unsurprisingly. Hints on Using xdump in Clients Let's explore how to dump a game. To make sense of a table, we need its meta-data, and to make sense of that table, we need meta-meta data. So we start with that: [3:640] Command : xdump meta meta XDUMP meta meta 1466920477 "name" 3 0 0 -1 "type" 1 0 0 33 "flags" 1 8 0 32 "len" 1 0 0 -1 "table" 1 0 0 26 /5 To interpret this table, we have to know the field names and their meanings. Clients hard-code them. They should be prepared to accept and ignore additional fields, and to cope with changes in field order, except they may rely on "name" coming first. A word on hard-coding. Clients hard-code *names*. The numbers used for table IDs and to encode symbols are none of the client's business. The encoding doesn't normally change within a game. Except when the game is migrated to a sufficiently different server. That's a rare event. Clients may wish to provide for such changes anyway, by decoupling the client's encoding from the server's, and dumping fresh meta-data on login. Incremental meta-data dump would be nice to have. So we don't know how symbol type and symbol set flags are encoded. To decode them, we need their symbol tables. However, we need flags and type only for tables we don't know, and there's one more table we do know, namely the table of tables. Let's dump that next, starting with its meta-data: [3:640] Command : xdump meta table XDUMP meta table 1466920477 "uid" 1 0 0 26 "name" 3 0 0 -1 /2 Because xdump table is referenced from elsewhere (xdump meta meta field table), the leftmost field must contain the key. Thus, the leftmost field's meta-data field table must be the table ID of xdump table itself. Indeed, its value matches the one we got in xdump meta meta. Let's try to dump the table: [5:640] Command : xdump 26 * XDUMP table 1466920477 0 "sect" 1 "ship" [...] 8 "nat" [...] 18 "sect-chr" 19 "ship-chr" [...] 26 "table" [...] /48 It worked! Now dump the two symbol tables we postponed. Because xdump accepts table IDs as well as names, we don't have to know their names: [5:640] Command : xdump meta 33 XDUMP meta meta-type 1466920477 "value" 1 0 0 -1 "name" 3 0 0 -1 /2 [6:640] Command : xdump 33 * XDUMP meta-type 1466920477 1 "d" 2 "g" 3 "s" /3 [7:640] Command : xdump meta 32 XDUMP meta meta-flags 1466920477 "value" 1 0 0 -1 "name" 3 0 0 -1 /2 [7:640] Command : xdump 32 * XDUMP meta-flags 1466920477 1 "deity" 8 "bits" 16 "hidden" /3 We now have complete meta-meta information: name type flags len table ----------------------------------------- name s (const) 0 type d (const) 0 meta-type flags d (bits const) 0 meta-flags len d (const) 0 table d (const) 0 table Dumping the remaining tables is easy: just walk the table of tables. Here's the first one: [7:640] Command : xdump meta 0 XDUMP meta sect 1466920477 "owner" 1 0 0 8 "xloc" 1 0 0 -1 "yloc" 1 0 0 -1 "des" 1 0 0 18 [...] /78 A whole load of tables referenced! Only one of them (not shown above) is a symbol table. owner references table nat. No surprise. xloc and yloc together reference the sector table, but that's not expressed in meta-data (yet). Let's stop here before this gets too long and boring. Experiment yourself! Check out example Perl code scripts/xdump.pl. Analysis of xdump as Configuration File Format xdump makes a lousy configuration format because it is unwieldy to edit for humans. That's because configuration files have different requirements than dumps: * Can be edited by humans with common tools, including text editors and spreadsheets. Using text editors requires a nice fixed-width table layout. Spreadsheet import requires trivial field separation. Tab character field separator or fixed width columns should do. The syntax should allow all that, but not require it. Trouble spots: - xdump's rigid horizontal and vertical spacing makes it impossible to align things visually. - xdump uses one line per record, which can lead to excessively long lines. - xdump's string syntax requires octal escape for space. - No comment syntax. * Each table is self-contained. You don't have to look into other tables to make sense of it. This conflicts with xdump's separation of data and meta-data. You need the table's meta-data to identify fields, and the referenced symbol tables to decode symbols. * Easy to parse. Don't compromise legibility just to please some dumb tool, though. Since we're trying to apply xdump to the configuration file problem, we get an additional requirement: * Reasonably close to xdump. Translation between machine-readable and human-readable should be straightforward, if meta-data is available. This leads to a human-readable dialect of the xdump language. Human-Readable xdump Language Fundamental difference to basic, machine-readable xdump: the rigid single space between fields is replaced by the rule known from programming languages: white-space (space and tab) separates tokens and is otherwise ignored. The space non-terminal is no longer needed. Rationale: This allows visual alignment of columns and free mixing of space and tab characters. Comments start with "#" and extend to the end of the line. They are equivalent to a newline. Rationale: Follow econfig syntax. Tables with a record UID in the leftmost field can be `split vertically' into multiple parts. Each part must contain the same set of records. The leftmost field must be repeated in each part. Other fields may be repeated. Repeated fields must be the same in all parts. Naturally, the parts together must provide the same fields as a table that is not split. Rationale: This is to let you avoid long lines. Line continuation syntax would be simpler, but turns out to be illegible. Requiring record UID is not technically necessary, as counting records works the same whether a table is split or not. Except humans can't count. Perhaps this should be a recommendation for use rather than part of the language. EBNF changes: * Header and footer: header = "config" identifier newline colhdr newline ; colhdr = { identifier [ "(" ( intnum | identifier ) ")" ] } [ "..." ] ; footer = "/config" newline ; If colhdr ends with "...", the table is continued in another part, which shall follow immediately. Rationale: - The xdump needs to identify itself as human-readable, hence change from "XDUMP" to "config". - The timestamp in the header is useless for the applications we have in mind for human-readable xdumps. The number of records in the footer is of marginal value at best, and a pain for humans to update. - The column header is due to the self-containedness requirement. It contains just the essential bit of meta-data: the column names. * Symbolic fields: field = intnum | flonum | string | symbol | symset ; Rationale: - Syntax for symbols and sets of symbols is due to the self-containedness requirement. Machine-readable xdump gets away with just numbers, which have to be decoded using meta-data. * Friendlier numbers and strings: flonum = ? floating-point in scanf %g format ? ; str-char = "\\" octal-digit octal-digit octal-digit | ? ASCII characters 32..126 except '"' and '\\' ? ; Rationale: - Machine-readable floating-point syntax is too rigid. Accept everything that scanf does. Could also change intnum to %i format, which accepts octal and hexadecimal in C syntax, but that seems not worth the documentation bother. - Machine-readable syntax requires \040 instead of space in strings to allow trivial splitting into fields. This is unacceptable here due to the legibility requirement, hence the change to str-char. * Parse nil as symbol: string = '"' { str-char } '"' ; Rationale: This is a technicality required to keep the parse unambiguous. * Symbols: symbol = identifier ; symset = "(" { symbol } ")" ; The special symbol "nil" is to be interpreted as null string. Rationale: - The symbol set syntax is the simplest that could work. We need to allow space between the symbols for legibility anyway, so why not make it the delimiter. A stop token is required to find the end of the field, and a start token is useful for distinguishing between symbol and symset. Bracketing with some kind of parenthesis is an obvious solution. The resulting sub-language for records is a superset of machine-readable sub-language for records. See src/lib/global/*.config for examples. Notes on Table Configuration Implementation econfig key custom_tables lists table configuration files. At this time, reading a custom table merges it with the built-in table, then truncates the result after the last record read from the custom table. Some of the tables are rather ugly in C, and cumbersome to edit. We thus moved them to configuration files (src/lib/global/*.config). The server reads them from builtindir before reading custom tables. The code dealing with these files is in src/lib/common/conftab.c. Actual work is done by src/lib/common/xundump.c, which accepts both human-readable and machine-readable input. The parser is not precise; it accepts human-readable syntax even within tables whose header marks them machine-readable. Symbolic index values in column headers are not implemented. They occur in item selector pkg, which is an array indexed by values in symbol table packing. Configuration tables contain values that are not meant to be customized. For instance, meta-data and symbol tables reflect the encoding of C language constructs in the server. Such selectors are marked (struct castr member ca_dump), so that the code can prohibit changes. All tables are checked against meta-data on server startup by ef_verify(). More elaborate checking would be nice, and probably requires additional meta-data. Appendix: Empire 3 C_SYNC --- A Cautionary Tale Clients are just as important as the server, and it's too darn hard to write a good client. In 1995, Ken Stevens decided to do something about it. Ken cast the problem as a data synchronization problem. Quote C_SYNC RFC 5.1, section `Abstract': This is a specification for a new method of synchronizing game data in the Empire client with data in the server. and section `Objectives': This new mode of communication between the server and the client will be called C_SYNC communication and will satisfy the following 6 criterea: (1) Output format will be version independent. So if someone is using an old EmpireToolkit, then it will still work with a newer version of the server. (2) Every C_SYNC message will be a self-contained packet. i.e. the client will not need to depend on previous messages (header messages) to determine the meaning of a C_SYNC message. (3) A C_SYNC message will be able to represent any of the player-accessible data that is contained in the server database (e.g. enemy ships, nations). (4) Bandwidth will be minimized (i.e. the format will be as concise as possible) while remaining human-readable (i.e. no binary messages). [Note that data compression may be added at a later date, but if it is added, it will be added on a separate port to maintain backwards compatability.] (5) The client will be able to tell the server whether it wants to receive C_SYNC messages and whether these messages can be sent asynchroniously (via "toggle sync" and "toggle async" respectively). (6) A portable ANSI C EmpireToolkit will be made available for parsing C_SYNC messages and managing the data they contain. C_SYNC worked by hooking into ef_write() & friends so it could `synchronize' the client on game state changes. Sounds jolly good, doesn't it? Well, it was a failure, and Wolfpack ripped it out right away. Quote the change log: Changes to Empire 4.0.0 - Initial release * Initial Wolfpack release - Long live the Wolfpack!!!! [...] * Removed C_SYNC. This is done for 2 reasons. 1) None of us like it or wish to support it. 2) We envision a better scheme for doing similar things will come along. But *why* did it fail? Just because Steve McClure hated it? Nope. C_SYNC failed for several different reasons, each of them bad, but only the last one is truly fundamental. a. Lack of a rigorous and complete definition. The RFC is long on syntax, but short on semantics. For instance, the unit type was encoded as a number. Unit characteristics happened to be dumped in an order that matched these numbers, but that wasn't defined anywhere. b. Overly complicated syntax. Trouble with encoding of strings. c. Buggy implementation. Malformed C_SYNC messages, duplicate messages, missing messages, semantically incorrect messages, you name it. d. Change of crew before it was finished. Wolfpack took over and understandable wasn't interested in this half-finished mess. None of the above is a fundamental, inherent flaw of the idea. The next one is more serious: e. It failed to achieve objective (4), and therefore slowed down clients too much to be of use in real-time combat. When you fired from a bunch of ships, C_SYNC would push complete records for all the ships and the target to you. Most of that data is redundant. That's because C_SYNC didn't transmit state changes, it resynchronized state, and the pieces of state it could transmit were too large. The network was slower then. But let's not be complacent. I/O is slow. Always was, most likely ever will be. Maybe sending the messages out of band (separate TCP stream) would help. Maybe not. And here comes the killer: f. The data to sync is not readily available on the server. Yup. Think about it. The game state on the server is *not* the same as on the client. The server grants the client a carefully limited view on certain parts of server game state on certain events. To be complete, a machine-readable protocol must disclose as much information as the human-readable output. Tracking server game state changes cannot do that alone. For instance, lookout tells you ship#, owner and location. That event does not trigger any state change on the server! To be correct, a machine-readable protocol must disclose no more information than the human-readable output. When you observe a server game state change, you can only guess what event triggered it, and what it disclosed to which player. You're stuck with conservative assumptions. That's the death knell for completeness. Correct assumptions will be non-obvious, so correctness is non-obvious, too, hence hard to achieve and maintain. Bottom line: tracking server state change cannot support a complete client protocol for hard theoretical reasons, and I believe it cannot support a correct one for practical reasons. Oddly enough, people criticized C_SYNC for all the flaws it had (and some it hadn't), except for f. What now? Throw up our hands in despair and give up? Nah. Ken tried a shortcut, and it didn't work. That doesn't mean there's no way at all. I believe the only way to get this done right is by tracking *events*. Whenever something is printed to a player, be it live connection or telegram, we need to transmit precisely the same information in machine-readable form. Much more work. xdump shares valuable ideas with C_SYNC, e.g. using selector meta-data. It is, however, much more modest in scope. We're pretty sure we can get it right and get it done in a reasonable time frame.