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| -rw-r--r-- | src/lib/cc/cc.dox | 107 |
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diff --git a/src/lib/cc/cc.dox b/src/lib/cc/cc.dox new file mode 100644 index 0000000..da8d79b --- /dev/null +++ b/src/lib/cc/cc.dox @@ -0,0 +1,107 @@ +// Copyright (C) 2016-2020 Internet Systems Consortium, Inc. ("ISC") +// +// This Source Code Form is subject to the terms of the Mozilla Public +// License, v. 2.0. If a copy of the MPL was not distributed with this +// file, You can obtain one at http://mozilla.org/MPL/2.0/. + +/** + @page libcc libkea-cc - Kea Configuration Utilities Library + +@section ccSimpleParser Simple JSON Parser + +Since the early beginnings, our configuration parsing code was a mess. It +started back in 2011 when Tomek joined ISC recently and was told to implement +Kea configuration handling in similar way as DNS Auth module. The code grew +over time (DHCP configuration is significantly more complex than DNS, with +more interdependent values) and as of Kea 1.1 release it became very difficult +to manage. The decision has been made to significantly refactor or even +partially rewrite the parser code. The design for this effort is documented +here: https://gitlab.isc.org/isc-projects/kea/wikis/designs/simple-parser-design. It discusses the original issues +and the proposed architecture. + +There are several aspects of this new approach. The base class for all +parsers is @ref isc::data::SimpleParser. It simplifies the parsers +based on DhcpConfigParser by rejecting the concept of build/commit +phases. Instead, there should be a single method called parse that +takes ConstElementPtr as a single parameter (that's the JSON +structures to be parsed) and returns the config structure to be used +in CfgMgr. An example of such a method can be the following: + +@code +std::pair<OptionDescriptor, std::string> +OptionDataParser::parse(isc::data::ConstElementPtr single_option) +@endcode + +Since each derived class will have the same parameter, but a different return +type, it's not possible to use virtual methods mechanism. That's perfectly +ok, though, as there is only a single instance of the class needed to parse +arbitrary number of parameters of the same type. There is no need to +keep pointers to the parser object. As such there are fewer incentives to have +one generic way to handle all parsers. + +@subsection ccSimpleParserDefaults Default values in Simple Parser + +Another simplification comes from the fact that almost all parameters +are mandatory in SimpleParser. One source of complexities in the old +parser was the necessity to deal with optional parameters. Simple +parser deals with that by explicitly requiring the input structure to +have all parameters filled. Obviously, it's not feasible to expect +everyone to always specify all parameters, therefore there's an easy +way to fill missing parameters with their default values. There are +several methods to do this, but the most generic one is: + +@code +static size_t +isc::data::SimpleParser::setDefaults(isc::data::ElementPtr scope, + const SimpleDefaults& default_values); +@endcode + +It takes a pointer to element to be filled with default values and +vector of default values. Having those values specified in a single +place in a way that can easily be read even by non-programmers is a +big advantage of this approach. Here's an example from simple_parser.cc file: + +@code +/// This table defines default values for option definitions in DHCPv6 +const SimpleDefaults OPTION6_DEF_DEFAULTS = { + { "record-types", Element::string, ""}, + { "space", Element::string, "dhcp6"}, + { "array", Element::boolean, "false"}, + { "encapsulate", Element::string, "" } +}; +@endcode + +This array (which technically is implemented as a vector and +initialized the C++11 way) can be passed to the aforementioned +setDefaults. That code will iterate over all default values and see if +there are explicit values provided. If not, the gaps will be filled +with default values. There are also convenience methods specified for +filling in option data defaults, option definition defaults and +setAllDefaults that sets all defaults (starts with global, but then +walks down the Element tree and fills defaults in subsequent scopes). + +@subsection ccSimpleParserInherits Inheriting parameters between scopes + +SimpleParser provides a mechanism to inherit parameters between scopes, +e.g. to inherit global parameters in the subnet scope if more specific +values are not defined in the subnet scope. This is achieved by calling +@code +static size_t SimpleParser::deriveParams(isc::data::ConstElementPtr parent, + isc::data::ElementPtr child, + const ParamsList& params); + +@endcode + +ParamsList is a simple vector<string>. There will be more specific +methods implemented in the future, but for the time being only +@ref isc::data::SimpleParser::deriveParams is implemented. + +@subsection ccMTConsiderations Multi-Threading Consideration for Configuration Utilities + +No configuration utility is thread safe. For instance stamped values are +not thread safe so any read access must be done in a context where write +access at the same time is not possible. Note that configuration is +performed by the main thread with service threads stopped so this constraint +is fulfilled. + +*/ |